Revert PR#131

recent changes for http gem breaks reel. it is good to keep the dependency on 0.5.x series.

websocket_parser gem has left http gem since v0.16.0 release. I believe that #131 was just needed to bump websocket_parser gem version.

Reel's master branch build started to fail after the PR tarcieri/http#80 because of the removal of lib/http/header.rb.

Build log for master HEAD (5ee0ae49ac76a8e45a)
https://travis-ci.org/unakatsuo/reel/jobs/19233177

```
$ bundle install
$ bundle exec rake
/home/travis/.rvm/rubies/ruby-2.0.0-p353/bin/ruby -S rspec ./spec/reel/connection_spec.rb ./spec/reel/http_server_spec.rb ./spec/reel/https_server_spec.rb ./spec/reel/response/writer_spec.rb ./spec/reel/response_spec.rb ./spec/reel/websocket_spec.rb
[Coveralls] Set up the SimpleCov formatter.
[Coveralls] Using SimpleCov's default settings.
/home/travis/build/unakatsuo/reel/lib/reel/response.rb:5:in `<class:Response>': uninitialized constant HTTP::Header (NameError)
	from /home/travis/build/unakatsuo/reel/lib/reel/response.rb:4:in `<module:Reel>'
	from /home/travis/build/unakatsuo/reel/lib/reel/response.rb:3:in `<top (required)>'
```