fix: require at least node v4 to run on travis (npm-run-all wants it and istanbul is failing to find _mocha)

Author: fluffynuts

.travis.yml

@@ -1,4 +1,5 @@
 language: node_js
+node_js: "4"
 sudo: false
 script: "npm run-script coverage"
 after_script: "cat ./coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js"