Avoid running tests if rebuild-script is making repository
dirty because this means that code was commited
without rebuilding first.

This prevents wasted time on reviews.

Per the [changelog](https://github.com/pypa/virtualenv/blob/6d8a4ed81f40223a3f9ea1ba5d11be500d9f667a/docs/changes.rst)
notes for `15.1.0`, Python 3.6 is supported. This change adds 3.6 to the
trove classifiers and drops the now-unsupported 3.3.

py33 reached EOL and the Travis version is broken
as it missing zlib encoder, making impossible
to run bin/build_script.py
py36 is officially released on all platform so
we need to test with it.

- with no arguments
- with --python=<fullpath>
- with -p <binary>

Also test creating a {2,3}-based venv with python {3,2}