Move dependencies from gemspec into Gemfile
Lock http_parser.rb dependency to ~> 0.6.0
Test in both 1.8 and 1.9 mode on JRuby
Test against Ruby 2.1.0
Add temporary workaround for failing build on Ruby 1.8.7

Rubygems broke builds on Ruby 1.8.7:

https://github.com/travis-ci/travis-ci/issues/1793
https://github.com/rubygems/rubygems/pull/763

This patch is a temporary workaround until rubygems is updated.