Implement CI on GitLab

It now works on jenkins.gnome.org; tested on Ubuntu 17.04 and Fedora (JFR it used to fail);

============================================================================
Testsuite summary for gjs 1.51.1
============================================================================
# TOTAL: 796
# PASS:  782
# SKIP:  14
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================

Patches to upload:

• 1: update CI to use SpyderMonkey 52
• 2,3: enable CI on GitLab (sharing the same scripts with Travis)

TODO:

• decide what to test: Ubuntu+Fedora? Each distro using gcc+clang?
• now, I'm using a clean build (build everything). Is that ok?
• it is slow (aprox 1 hour to run the above 4 tests). Not sure yet if I can avoid the clean build (disk space on GNOME machine to keep the cache?)
• any crazy stuff such as: packaging, more tests, etc.

The possibilities on GitLab are (almost) endless (since your hardware can be attached; and you do it).