Use fetch.prune for automatic pruning

No need to call git again, since 1.8.5, there has been a fetch.prune
option for automatically pruning the remotes when fetching.
Add git-shortlog aliases
Remove liftoff from default gems

Liftoff is distributed via Homebrew now, so this isn't needed.
Speed up rbenv startup

- Don't force a rehash on launch
- Still don't force a rehash on launch
Don't fuck up the MANPATH

Seriously bro, what the hell
Improve prompt speed
Set filetype for pull requests
Add words-to-avoid vim plugin
Add vim-hardtime

This is probably a horrible idea, but it would be nice to break these
bad habits.