raven-9999: Convert the old 2.0.10 ebuild into a live ebuild

raven-9999: Convert the old 2.0.10 ebuild into a live ebuild

That also serves double-duty as a 5.0.0 ebuild, since the changes
between 5.0.0 and the current master don't require different install
instructions.

Upgrade changes:

* Modernize PYTHON_COMPAT entries.  The upstream package is compatible
  with all of these (or at least tries to test on all of these [1]).
  They test on 2.6 too, but that's not in the Portage tree anymore.
* I've added an if block to swap in git-2 live ebuild handling
  depending on the package version.
* I've dropped the runtime dependencies, because the only runtime
  dependency is simplejson, which I imagine they only use for Python
  2.6 support [2].
* The new dependencies are almost straight from the 5.0.0 setup.py,
  except I don't include python-coveralls (and neither will the next
  version of setup.py [3]).  You'll want to emerge it by hand if
  you're using 5.0.0, to avoid Setuptools trying to install it for
  you.

[1]: https://github.com/getsentry/raven-python/blob/5.0.0/.travis.yml
[2]: https://github.com/getsentry/raven-python/blob/5.0.0/docs/install/index.rst#requirements
[3]: https://github.com/getsentry/raven-python/pull/497
     9b5c3d3d (setup.py: Remove python-coveralls requirement,
     2014-09-30)