Add ag.vim plugin

It does some fancier stuff than I was doing by replacing the grepprg.
Remove little used greplace plugin

I generally just use the the `replace` shell command from thoughtbot.
Enable spell checking in git commit messages
Use thoughtbot's vim-rspec plugin

It now has support for dispatch and a few other improvements over what I
had locally. Trying out some non-standard mappings for these.
Add vim-bundler and vim-rake

Not positive I will use these, but I want to try. And who doesn't want
more tpope in their vim?
Get rid of 'visual' search and replace shortcut

Not really sure what this is does. So let's get rid of it.
Don't remember cursor position on commit messages

... that was obnoxious.