Changes to Gulps own implementation of code completion.

Hi!

There is already an implementation of this within Gulp. But I think adding this wrapper to the oh-my-zsh repo as with npm is a good idea.

I've changed this plugin to use the Gulp implementation with "gulp --completion". The current gulp implementation is somewhat similar to your own, but as of 3.7.0 they change to use a new "--tasks-simple" flag, which should be quicker I figure.