Update CMakeLists.txt

This commit includes checking whether ../data directory (from build directory) and ../../stk-assets directory (from build directory) exists or not. If they don't exist, then it gives error message for the corresponding directory. I have included the code for this on line 320.
Update CMakeLists.txt
Merge branch 'master' of https://github.com/abhishek1995/stk-code into abhishek1995-master
Let cmake test if stk-assets exists in the expected place. If not,
cmake is aborted with an error message. The option -DCHECK_ASSETS=off
option can be used to disable this test. The latter is also used
for travis.