General: node 7 for testing

.travis.yml

 language: node_js
 node_js:
-  - stable
+  - "7"
 script:
   - npm run lint && npm run test && npm run downloadsolc && npm run make-mock-compiler && npm run build
   - ./ci/browser_tests.sh

README.md

@@ -53,6 +53,8 @@ Run the unit tests via: `npm test`
 For local headless browser tests run `npm run test-browser`
 (Requires selenium to be installed - can be done with `npm run selenium-install`)
 
+Running unit tests via `npm test` requires at least node v7.0.0
+
 ## Browser Testing
 
 To run the Selenium tests via Nightwatch serve the app through a local web server: