Switch Gist.vim from a subtree to a submodule

I've decided I'm not a fan of subtrees. I don't think I'll always prefer
submodules over subtrees, but in this particular instance I do.

This reverts commit 635af89463541915752d3f1ba94fe832675da876, reversing
changes made to 1871bd35ac930f82ef312cc3f479233212539d8a.
Update submodules