WIP global: travis fixes

* Adds the language key (otherwise, [it's ruby](http://docs.travis-ci.com/user/getting-started/#Step-three%3A-Add-.travis.yml-file-to-your-repository)).
* Downloads the devscripts where they want to be downloaded.
* Moves `apt-get update` to `before_install` as [advised](http://docs.travis-ci.com/user/build-configuration/#Build-Lifecycle)
* Enables the Python 2.6 build, even if it doesn't look like it.

**blockers:**
- [x] tiborsimko/invenio-devscripts#17
- [x] bibknowledge requires libxml2 (#1879)
I guess this can now be changed back to the canonical location.  (`https://github.com/tiborsimko/invenio-devscripts` for now)