Update required http_parser.rb gem version to prevent collision with em-http-request

This change fixes version mismatch:

    Bundler could not find compatible versions for gem "http_parser.rb":
      In Gemfile:
        em-http-request (>= 0) ruby depends on
          http_parser.rb (>= 0.6.0) ruby

        goliath (>= 0) ruby depends on
          http_parser.rb (0.6.0.beta.2)