Remove unused `git_colour`

The `git_colour` variable used to set the colour of the Git info part of
PS1, but hasn't been used in ages.  Remove it.
Change CentosVM prompt colours

The CentosVM prompt colours would put multiple parts in red next to each
other if the current directory wasn't part of a Git repository.  That's
not helpful.  Don't do that.
Don't specify an explicit path to vim

When setting the VISUAL variable, just specify "vim" rather than giving
an explicit path.  Whichever one is in $PATH is probably the one we want
to use.  That's certainly the case on the current CentosVMs.
Add formatoptions settings to vimrc

Insert comment leaders when hitting Enter within a comment block in
insert mode, or when pressing o/O in normal mode.

Brings the vimrc closer in line with the version on the master branch.