Make sure package list is up to date before we start anything else

This should resolve the issue encountered in https://travis-ci.org/fabioz/Pydev/jobs/12286695 where apt tried to install an old package.

You should see in the log of the build of this branch[1] that instead of python-django_1.3.1-4ubuntu1.7_all.deb being fetched, python-django_1.3.1-4ubuntu1.8_all.deb is used for the install. 

This happened to work in earlier builds because either the 1.7 version was the current version or the mirror(s) had not purged it yet.

[1] https://travis-ci.org/jonahkichwacoders/Pydev/jobs/12293496