lock down cookiejar version

Current version of `cookiejar` is [broken](https://github.com/dwaite/cookiejar/issues/13). `em-http-request` fetches the latest version of `cookiejar` and breaks on require. 
```
irb(main):001:0> require 'em-http-request'
LoadError: cannot load such file -- cookiejar
        from /usr/local/Cellar/ruby/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/Cellar/ruby/2.0.0-p247/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
        from /usr/local/Cellar/ruby/2.0.0-p247/lib/ruby/gems/2.0.0/gems/em-http-request-1.1.2/lib/em-http/client.rb:1:in `<top (required)>'
```
To reproduce, run `sudo gem uninstall cookiejar em-http-request && sudo gem install em-http-request && ruby -e "require 'em-http-request'" `