Use fixed MAD-X revision for Travis tests

[coldfix]

Broken MAD-X revisions break pymad builds on Travis even if the pymad code is perfectly fine.

The natural way to avoid this is to use a fixed SVN revision (latest dev release?) for all Travis builds. There are two more advantages:

• make the stable configuration [pymad revision -> MAD-X revision] explicit
• can decrease Travis build times if using a prebuilt MAD-X version (maybe I will host a PPA)

I'd suggest to put the metadata in a YAML/JSON file. JSON is preferred if we want to use the information in setup.py (before installing pyyaml).