MAINT: Remove outdated version checks.

Because Numpy 1.8.0 will no longer supports Python versions < 2.6 we
no longer need to check for that and can also remove the code that is
specific to those earlier versions.

To make this a bit safer, the toplevel setup.py file now contains a
check of the Python version number and raises an error when run by an
unsupported version.
Merge pull request #3516 from charris/remove-outdated-version-checks

MAINT: Remove outdated version checks.