install wxgtk 3.0 and fall back to wxgtk 2.8 if it doesn't exist

This way 2.8 will automatically be used for Ubuntu 12.04 and 3.0 for 14.04 or higher.

By the way if you want to make the build-deps list smaller you can remove some of the boost libs that are installed as dependencies. Adding these will be enough: libboost-dev (>= 1.46), libboost-filesystem-dev (>= 1.46), libboost-regex-dev (>= 1.46), libboost-date-time-dev (>= 1.46)