Install MAD-X from a PPA

To reduce build times drastically. Furthermore the tests are now
executed against a fixed MAD-X revision, so daily changes to the
upstream won't break tests anymore.
Document usage of libmadx-dev package for Travis
Fix bug in setup.py

Sometimes pkg_resources may be available without having setuptools
installed (observed on ubuntu 12.04 LTS default installation). So better
check for setuptools directly.
Merge branch 'travis-use-ppa'

Speeds up build times from about 8 minutes to about 3 minutes.