pexif-9999.ebuild: Convert from thumbor-pexif to pexif

Now that pexif has cut 0.15 [1,2].

Also bump from EAPI 5 to 6 and from git-2 to the newer git-r3 (there
were no changes impacting this ebuild [3]).

[1]: https://github.com/thumbor/thumbor/issues/294
     Subject: As pexif is now on pypi revert from thumbor fork
[2]: https://pypi.python.org/pypi/pexif/0.15
     Uploaded on: 2014-12-03
[3]: https://devmanual.gentoo.org/ebuild-writing/eapi/#eapi=6

thumbor-pexif-0.14.1.ebuild: Dead ebuild for thumbor (which wants <1.0)

dev-python/thumbor-pexif: Add this thumbor dependency

I've submitted the new homepage upstream [1].  This package is old,
and not Python 3 compatible, with stuff like:

  print >> sys.stderr, usage

[1]: https://github.com/thumbor/pexif/pull/1