remove dev_dependency on self

Performing a `bundle install` resulted in the following warning: 

> Your Gemfile lists the gem em-websocket (>= 0) more than once.
> You should probably keep only one of them.
> While it's not a problem now, it could cause errors if you change the version of just one of them later.

I found it odd that the gemspec has a dev dependency on itself.  Removing the dev dependency removed the above message.