[IMP] remove setting of color.ui

After further considering PR #21's arguments, the setup script should
not go around setting `color.ui`:

* it shouldn't be needed for recent gits with default compile
  settings[0]
* it may override advanced users's own colour configuration/
  customisations (or disabling)
* it forces default colours on users for which it may be useless
  (colour- blind) or even detrimental (other vision impairments)
* some linux distributions apparently default to idiotic pagers which
  can't handle ANSI colour codes by default and spew garbage to the
  terminal

[0] https://github.com/git/git/blob/master/Documentation/RelNotes/1.8.4.txt#L178-L180