Fix travis build and some gdl CMake checks.

 - Use Ubuntu 14.04 to test build (run distribution upgrade before test).
 - Add gtk+-3.10 support for valac 0.20.
    - Use vapidir option to overwrite system vapis (instead of custom vapi
      which may conflict with system vapi coming as dependency).
    - Use own directory for each extra vapi to not pull in all vapis.
 - Support old gdl versions again (fix regression introduced by 9177d13).
CMake: Search for global valac first.

 - Fix regression introduced by 19347ef. Don't try to find a smarter solution
   than the OS.
Update versioned dependencies.

 - gtk+-3.0 and gtksourcview-3.0 >= 3.10 (from 3.4)
Update Valama vlp file.

 - Set defines according to CMakeLists.txt.
 - Update extra vapi paths.