[vim] solarized_termtrans to prevent colour issue

Enable `solarized_termtrans` to avoid colour issues when using Vim in
iTerm2 on Mac OS X with the 'Solarized' colour theme.

See the documentation at:
http://ethanschoonover.com/solarized/vim-colors-solarized
[iTerm2] Check in iTerm2 configuration

See the 'Load preferences from a custom folder or URL' setting in the
'General' tab of the iTerm2 preferences.