Add proper yarn support for travis

https://docs.travis-ci.com/user/languages/javascript-with-nodejs#Travis-CI-supports-yarn

.travis.yml

@@ -3,14 +3,14 @@ node_js:
   - "0.12"
   - "4"
   - "6"
-install: NV=$(node --version); if [ ${NV:0:5} = "v0.12" ]; then npm install; else npm install -g yarn; yarn --pure-lockfile; fi
 script:
   - npm run build
   - npm run lint
   - npm run pretest && npm run test:only
   - npm run codecov
 after_success:
-- bash <(curl -s https://codecov.io/bash)
+  - bash <(curl -s https://codecov.io/bash)
+cache: yarn
 notifications:
   email:
     on_success: never