Remove unnecessary dependencies and add a cache for the build files.
Additionally limit Git clone depth and use build matrices properly.
The CMake package dependency was removed since Travis comes with 3.2
and the installer only brings in 3.5, while the current minimum is 2.6.
Add support for mtime_cache, a small Ruby gem for hashing file mtime
and automatically updating unchanged files so that CMake knows which
cached build files are still valid since Git doesn't track mtime.
Going forward it will be alot easier to maintain.
TODO: simplify path definitions
Conflicts:
src/servers/Server_Lobby/CMakeLists.txt
src/servers/Server_REST/CMakeLists.txt
src/servers/Server_Zone/CMakeLists.txt