Small tweaks to prompt:
  - replace '[]' with '()'
  - command line starts on new line
Workaround gnome-terminal identification changes.

gnome-terminal (current preferred terminal) 3.12+ no longer self
identifies in COLORTERM. COLORTERM is left empty. If COLORTERM is empty
then check to see if this is gnome-terminal so that colors (etc...) can
be set correctly.
Fix typo when setting secondary Vim colorscheme.