Update CtrlP.

The new maintainer has incorporated my tag centering patch, so we no
longer need to patch CtrlP.
Update UltiSnips.

My upstream patch has been accepted, so we no longer need to patch it
locally.
Switch BufMRU mapping to <C-6> instead of <Space>.
Switch sneak plugin keymap to <Space>s and <Space>S.
Merge branch 'upgrade-ctrlp-and-ultisnips' of https://github.com/jszakmeister/vimfiles