gitlab: added gitlab continuous integration

1 job for master in 17 seconds (queued for 1 second)
Name Stage Failure
build Build
Setting up ninja-build (1.7.2-1) ...
Setting up libjsoncpp1:amd64 (1.7.4-3) ...
Setting up cmake (3.7.2-1) ...
Processing triggers for libc-bin (2.24-11+deb9u3) ...
$ mkdir build && cd build
$ cmake -G Ninja
CMake Error: The source directory "/builds/gauthier.quesnel/bits/build" does not appear to contain CMakeLists.txt.
Specify --help for usage, or press the help button on the CMake GUI.
ERROR: Job failed: exit code 1