MAINT: Update INSTALL.txt after dropping Python 2.4, 2.5.

Make the required Python versions >= 2.6 or 3.2. Remove text specific
to earlier Python versions.
MAINT: Improve INSTALL.txt wording and wrap lines at 75 characters.
Merge pull request #3271 from charris/maint-update-install-txt

MAINT: Update INSTALL.txt after dropping Python 2.4, 2.5.