Use tpope's fork of the original _gem completion gist.

The _gem completion for zsh used previously was based on this [gist](https://gist.github.com/alexvollmer/164465) by Alex Vollmer. 

I found that this completion script was incredibly slow on my machine for some reason and it always failed when you try tab completing the first time. I used tpope's fork and rather than me trying to explain what the changes are (I'm not too familiar with zsh's completion system), tpope has explained what he has done here: [link](https://gist.github.com/alexvollmer/164465#comment-47689)