Prevent installing Review Board 3.0 on Python 2.6 or lower.
Prevent installing Review Board 3.0 on Python 2.6 or lower.

Review Board 3.0 requires Python 2.7 or higher. This change updates our
setup.py to prevent users from accidentally installing it on Python 2.6.

Previously, we had checks for different version ranges, recommending
different versions of Review Board. We no longer want to recommend
anything older than 2.5, as of this release, so those checks and
messages have been updated.

Testing Done:
Tested trying to build with Python 2.6 and 2.7.

Reviewed at https://reviews.reviewboard.org/r/8606/

chipx86/reviewboard,chipx86/reviewboard,brennie/reviewboard,sgallagher/reviewboard,brennie/reviewboard,brennie/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,davidt/reviewboard,davidt/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,davidt/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,brennie/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,davidt/reviewboard