Update http_parser.rb to 0.6.0.beta.2

This version brings both bugfixes and performance improvements for JRuby.

It also lets hatetepe bundle side-by-side with Reel 0.4.0 inside
webmachine-ruby. At present the two gems conflict.
Merge pull request #5 from tarcieri/upgrade-http-parser

Update http_parser.rb to 0.6.0.beta.2