Rename gunicorn-python2 to python2-gunicorn

This package installs an executable named `gunicorn-python2`, and the package
was originally named after the executable itself. However, an examination of
other packages shows that the "python2" portion of the name should come first,
regardless of the executable's name:

* python2-flake8 installs `flake8-python2`
* python2-pyflakes installs `pyflakes-python2`
* python2-markdown installs `markdown_py2`

This change is also in line with what the wiki suggests.

Make python2-gunicorn provide and conflict with gunicorn-python2.