ci(Travis): Use yarn instead of npm. Drop old node versions

Author: nodkz

.travis.yml

@@ -1,24 +1,20 @@
 sudo: false
 language: node_js
 cache:
+  yarn: true
   directories:
     - node_modules
 notifications:
   email: true
 node_js:
+  - "7"
   - "6"
   - "5"
   - "4"
-  - "0.12"
-  - "0.10"
-before_install:
-  - npm i -g npm@^2.0.0
-  - npm i graphql graphql-compose
-before_script:
-  - npm prune
 script:
-  - npm run coverage
-  - npm run build
+  - yarn add -g graphql graphql-compose
+  - yarn run coverage
+  - yarn run build
 after_success:
   - 'curl -Lo travis_after_all.py https://git.io/travis_after_all'
   - python travis_after_all.py