Rakefile: Remove matchit.
gvimrc: Remove NERDTree functions.

These were causing crashes when editing dotfiles. For instance, any time
I tried to :Edit .env, MacVim would crash.
vimrc: Ignore node_modules.
vimrc: Treat .es6 files as JavaScript.
ackrc: Ignore bower and node directories.
Rakefile: Add vim-sensible.
vimrc: Silence pathogen warnings.