Restyle .travis.yml.

* Update travis.yml to rely on a basic set of commands, and not on those shitty
  build scripts.
* Remove useless commands in the script section.
* Re-order other commands, like `apt-get update` in before_install (as suggested
  by travis doc), `apt-get install` only in the install directive, etc.

* Install debian dependencies, not pip ones.

* Add equivs as dependency for build, in order to use `mk-build-deps`