Enhancements to README and travis tests

HSPHP passes all tests on php 5.5, 5.6, and HHVM so I figured they should be added to travis. [Here's my fork on travis](https://travis-ci.org/Andrewpk/HSPHP).

I searched for handlersocket on packagist and HSPHP didn't show up - which prompted these small enhancements. Should be an easy merge as long as you find the changes appropriate/acceptable.

Added a bunch of keywords in composer.json and cleaned up the README file (added syntax highlighting as well).

Thanks for the library!