[12391] Remove the PREFIX parameter from the CMake files and use the standard CMAKE_INSTALL_PREFIX parameter instead

This also fixes the behaviour with relative paths, until now, it
calculated the absolute path from the location of the top level
CMakeLists.txt instead of your current directory where you called cmake.

You need to re-run CMake and define CMAKE_INSTALL_PREFIX and remove
PREFIX! You can do so by either removing all files in your build directory
and start from scratch or run 'cmake -DPREFIX= -DCMAKE_INSTALL_PREFIX=<install_path> <path_to_source>'.
A check has been added to the CMake file to make sure that PREFIX is no
longer used.
[12392] Add copyright header and some comments to .travis.yml

[ci skip]
[12393] reduce clone time in Travis by only getting the latest commit and not the whole history (default for travis is 100)