chore(npm/travis): upgrade to npm 2.5 and require it via package.json

currently karma's dependencies don't install on node 0.12 or io.js so we'll just update npm in hope that that
this will mitigate "cb() never called!" erorrs. See: https://travis-ci.org/angular/angular.js/jobs/51474043#L2181

Author: IgorMinar

.travis.yml

@@ -44,6 +44,7 @@ install:
   - npm config set spin false
   # Log HTTP requests
   - npm config set loglevel http
+  - npm install -g npm@2.5
   - time npm install
 
 before_script:

package.json

@@ -6,6 +6,11 @@
     "type": "git",
     "url": "https://github.com/angular/angular.js.git"
   },
+  "engine": {
+    "node" : "~0.10",
+    "npm": "~2.5"
+  },
+  "engineStrict": true,
   "devDependencies": {
     "angular-benchpress": "0.x.x",
     "benchmark": "1.x.x",