Add GitLab CI script
As suggested and merged for tor I'd like to propose a GitLab CI script for Tor Browser. My intention to be able to build
tor-browser-bundle locally which I tried for some time and failed because of error which I hope will be brought to the attention of devs automatically rather than after painful reports.
The script currently uses Ubuntu 17.04 only, but since GitLab CI is based on Docker images, the number of possible OS to test on is basically infinite.