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:**
- [ ] tiborsimko/invenio-devscripts#17
- [x] bibknowledge requires libxml2 (#1879)
> sudo pip install -U pip

IIRC it wasn't updating itself properly, hence the `curl` workaround.  But since you touch only Debian, it should be OK, I guess.  I'll try to retest with older Debian images.