Python 2.6 on Travis CI

Currently python 2.6 is _“enabled”_ but there is only one job per build on travis ([build 551](https://travis-ci.org/jirikuncar/invenio/builds/21755497)) when two should be there ([build 48](https://travis-ci.org/greut/invenio/builds/21758555)).

The good thing about the `language: python` is that it comes with a _virtualenv_ already; much simpler indeed.

BTW: to fix the 2.6 build, moving `importlib` above `Flask-Collect` in requirements.txt will probably help.