Remove duplicated dependency in gemspec file

I got this error when running `bundle update`

    Using http_parser.rb (0.6.0)

    em-websocket at /home/davidr/.rvm/gems/ruby-2.1.0/bundler/gems/em-websocket-bd716eda8b2d did not have a valid gemspec.
    This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
    The validation message from Rubygems was:
      duplicate dependency on eventmachine (>= 0, development), (>= 0.12.9) use:
        add_runtime_dependency 'eventmachine', '>= 0', '>= 0.12.9'

    Using em-websocket (0.5.0) from git://github.com/igrigorik/em-websocket.git (at master)

This fixes it.