Update Gemfile for a later version of eventmachine

Fixes problems encountered related to old version of eventmachine when running `bundle exec rake server` (https://github.com/eventmachine/eventmachine/issues/389)

(rubyeventmachine.so: undefined symbol: rb_enable_interrupt)

NB: you probably don't want to merge this patch directly, as you will want to update the Gemfile.lock simultaneously by running `bundle update` locally. Feel free to use this fix to assist in fixing this issue.