Move from npm cli -> yarn

yarn is a new cli for the npm registry from Facebook, Google, and Tilde.
It's patterned after Cargo, and @wycats was involved in making it. The
big things we care about here:

1. It is faster. 30s vs 45 for npm cli on my mcahine.
2. It has a lockfile. Cargo has a lockfile. Lockfiles are good.

This commit moves development over to yarn.

Some changes:

Instead of needing to `npm install -g` things, we can instead include
them in our `package.json`, and use `yarn run`. This doesn't pollute the
global environment, which is a win!

The README and .travis.yml are updated accordingly.

Author: steveklabnik

.travis.yml

@@ -4,27 +4,26 @@ sudo: false
 
 cache:
   directories:
-    - node_modules
+    - $HOME/.yarn-cache
 
 before_install:
   - nvm install 4.6.0
   - export PATH=/usr/local/phantomjs-2.0.0/bin:$PATH
-  - "npm config set spin false"
-  - "npm install -g npm@^2"
+  - npm config set spin false
+  - npm install -g yarn
 
 install:
   - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
-  - npm install -g bower
-  - npm install
-  - bower install
+  - yarn
+  - yarn run bower install
 
 before_script:
   - psql -c 'create database cargo_registry_test;' -U postgres
 
 script:
   - cargo build
   - cargo test
-  - npm test
+  - yarn run ember test
 
 after_success:
   - travis-cargo coveralls --no-sudo

README.md

@@ -9,20 +9,20 @@ This project is built on ember-cli and cargo, visit
 
 ## Working on the Frontend
 
-```
+```bash
 git clone https://github.com/rust-lang/crates.io.git
 cd crates.io/
-npm install
-npm install -g ember-cli bower
-bower install
+npm install -g yarn # if you don't have yarn
+yarn
+yarn run bower install
 ```
 
 The website's frontend is built with [Ember.js](http://emberjs.com/). This 
 makes it possible to work on the frontend without running a local backend.
 To start the frontend run:
 
 ```bash
-npm run start:staging
+yarn run start:staging
 ```
 
 This will run a local frontend using the staging backend (hosted on Heroku at 
@@ -37,9 +37,9 @@ specify arguments to `npm start`. For example you can set the proxy to
 To do this, run:
 
 ```bash
-npm start -- --proxy https://crates.io
+yarn start -- --proxy https://crates.io
 # or
-npm run start:live
+yarn run start:live
 ```
 
 **Note**: This requires npm 2.
@@ -52,8 +52,8 @@ phantomjs-prebuilt`.
 Then run the tests with:
 
 ```
-ember test
-ember test --server
+yarn run ember test
+yarn run ember test --server
 ```
 
 ## Working on the Backend
@@ -97,7 +97,7 @@ follows:
 6. **Optionally** start a local frontend:
 
     ```
-    npm run start:local
+    yarn run start:local
     ```
 
 ### Running Tests

bower.json

@@ -1,7 +1,9 @@
 {
   "name": "cargo",
   "dependencies": {
+    "Faker": "~3.0.0",
     "ember": "~2.6.0",
+    "ember-cli-moment-shim": "~0.1.0",
     "ember-cli-shims": "0.1.1",
     "ember-cli-test-loader": "0.2.2",
     "ember-load-initializers": "0.1.7",
@@ -10,13 +12,11 @@
     "ember-resolver": "~0.1.20",
     "jquery": "1.11.3",
     "loader.js": "ember-cli/loader.js#3.4.0",
-    "qunit": "~1.20.0",
-    "ember-cli-moment-shim": "~0.1.0",
+    "lodash": "~3.7.0",
     "moment": ">= 2.8.0",
     "moment-timezone": ">= 0.1.0",
     "normalize-css": "~3.0.3",
     "pretender": "~1.1.0",
-    "lodash": "~3.7.0",
-    "Faker": "~3.0.0"
+    "qunit": "~1.20.0"
   }
 }

package.json

@@ -7,12 +7,12 @@
     "test": "tests"
   },
   "scripts": {
-    "build": "ember build",
-    "start": "ember server",
-    "start:local": "ember server --proxy http://127.0.0.1:8888",
-    "start:staging": "ember server --proxy https://staging-crates-io.herokuapp.com",
-    "start:live": "ember server --proxy https://crates.io",
-    "test": "ember test"
+    "build": "yarn run ember build",
+    "start": "yarn run ember server",
+    "start:local": "yarn run ember server --proxy http://127.0.0.1:8888",
+    "start:staging": "yarn run ember server --proxy https://staging-crates-io.herokuapp.com",
+    "start:live": "yarn run ember server --proxy https://crates.io",
+    "test": "yarn run ember test"
   },
   "repository": "",
   "engines": {
@@ -21,6 +21,7 @@
   "author": "",
   "license": "MIT",
   "devDependencies": {
+    "bower": "^1.7.9",
     "broccoli-asset-rev": "^2.4.2",
     "ember-ajax": "^2.0.1",
     "ember-cli": "2.8.0",