Removed tests for different python versions.

These tests really only tested zc.buildout's internal script generation, which
supported different python versions. It wasn't djangorecipe functionality.

Buildout 2.0 removed the support for different versions, btw.
Using sys.executable instead of the same one passed by buildout.

Buildout 2.0 doesn't pass the executable anymore as it only supports a single
python executable.