Added gitlab-ci configuration file.

Also removed automatic installation of dependencies (done in gitlab-ci
pipeline instead).
2 jobs for master in 10 seconds (queued for 1 second)
Name Stage Failure
failed
testsuite_octave Test
Cloning repository...
Cloning into '/home/gitlab-runner/builds/a354a37e/0/Dynare/dates'...
Checking out 54e05d16 as master...
Updating/initializing submodules...
$ git clone https://git.dynare.org/Dynare/m-unit-tests
Cloning into 'm-unit-tests'...
$ cp -r binaries/linux externals/x13
cp: cannot stat 'binaries/linux': No such file or directory
ERROR: Job failed: exit status 1