Update almost all Pombola Python dependencies

These are updates suggested by Gemnasium. Some problematic packages have
been left back, e.g. argparse (see the notes in requirements.txt)

Also, the upgrade of Django to 1.7 has been left out, since needs some
time working carefully through the release notes.

pyelasticsearch has been switched to elasticsearch, which is now
required instead by django-haystack since v2.2.0 (f0a6e90b06f)