Add Node.js 4.0 stable to Travis config
Add Node.js 4.0 stable to Travis config

NodeJS v4 was just released [NodeJS v4 announcement](https://nodejs.org/en/blog/release/v4.0.0/). This updates the CI build script to accommodate the release.
alanev/postcss,OEvgeny/postcss,bendtherules/postcss,omeripek/postcss,asan/postcss,mdegoo/postcss,wjb12/postcss,MadLittleMods/postcss,kk9599/postcss,bezoerb/postcss,qiucanghuai/postcss,smagic39/postcss,mshahidkhan/postcss,eprincev-egor/postcss,marcustisater/postcss,johnotander/postcss,andrepolischuk/postcss,postcss/postcss,pascalduez/postcss,alexmchardy/postcss,Shugar/postcss,XOP/postcss,Paddy-Hamilton/postcss,postcss/postcss,KivyGogh/postcss,marek-saji/postcss,mahtd/postcss,LucasFrecia/postcss,jonathantneal/postcss,maximkoretskiy/postcss,jedmao/postcss,Semigradsky/postcss,dehuszar/postcss