remove dev_dependency on self

[stve]

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.

[mloughran]

Thanks