ENH: improve handling of enstaller in the requirement list.

The behaviour is now as follows:

 - if 'enstaller' is in the requirement, it is removed from the list
 - if 'enstaller-$version' is specified, then a warning is printed about
   downgrade
STY: ensure needs_to_downagrade_enstaller always return a bool.
STY: fix typo/capitalization.
Merge pull request #160 from enthought/handle_enstaller_in_requirement_v2

ENH: improve handling of enstaller in the requirement list.