Remove the requirement to use --enable-stl build of wxWidgets.

Kinda overestimated Linux distros progress there. Oh well.
Setup Travis CI builds.

Instead of only verifying the translations, as was done until now,
actually run a full Linux build to test building and packaging.

Because Poedit's dependencies are a bit too modern for the ancient
Ubuntu 12.04 LTS used by Travis, rely heavily on a PPA with the deps.