fix(travis): ensure babel is run for each node version

.travis.yml

@@ -11,14 +11,19 @@ cache:
   directories:
     - node_modules
 
+before_script:
+  - npm prune
+
 after_success:
   - curl -Lo travis_after_all.py https://git.io/travis_after_all
   - python travis_after_all.py
   - export $(cat .to_export_back) > /dev/null
   - npm run coverage
   - npm run codeclimate
-  - declare -A PRESETS=([es2015]= ["node4"]=node4 ["node5"]=node5 ["node6"]=node6);
-  - for preset in "${!PRESETS[@]}"; do BABEL_ENV=$preset npm run compile -- -d lib/${PRESETS[$preset]}; done
+  - BABEL_ENV=es2015 npm run compile -- -d lib
+  - BABEL_ENV=node4 npm run compile -- -d lib/node5
+  - BABEL_ENV=node5 npm run compile -- -d lib/node4
+  - BABEL_ENV=node6 npm run compile -- -d lib/node6
   - npm run semantic-release
 
 branches: