chore(ci): add node v10 to test matrix (#69)

* Update to node 10 in .travis.yml

* chore(ci): add `ignore-engines` flag to `yarn`

Author: medfreeman

.travis.yml

@@ -1,6 +1,7 @@
 language: node_js
 node_js:
-- '8'
+  - '8'
+  - '10'
 matrix:
   fast_finish: true
 cache:
@@ -10,6 +11,7 @@ cache:
   - node_modules
 before_install:
 - yarn config set no-progress
+- yarn config set ignore-engines true
 - yarn global add greenkeeper-lockfile@1
 install:
 - yarn