Set $EDITOR to current Vim

The 'vim-vinegar' plugin seems to require Vim 7.4. Darwin 13.0 ships
with Vim 7.3. Set $EDITOR to the current Vim instead of the system Vim.
vim: Fix backspace

The system Vim on Darwin ships with these settings. Apply them to the
current Vim too.
vim: Do not set Octave file type

This is handled by the 'vim-octave' plugin.
vim: Do not set Gemfile file type

This is handled by the 'vim-ruby' plugin.
vim: Do not set Vagrantfile file type

This is handled by the 'vim-ruby' plugin.
Add 'vim-json'