Remove commentary Vim plugin

I don't think I've ever used this.
Remove Gist Vim plugin

I don't think I've ever used this.
Remove Vim plugin expand-region

I don't think I've ever used this.
Remove Vim nerdtree plugin

I don't think I've ever used this.
Update Vim Powerline to latest version

This repo apparently doesn't have anything in its master branch, so I
checked out its develop branch where all the goods are located.