Added CI configuration

This merge adds a gitlab CI configuration for this project, which checks that the packages builds on the follow systems

• python3.6 (setup.py sdist bdist_wheel)
• debian8 and debian9 (dpkg-buildpackage)
• rhel7 (rpmbuild)

and then runs a basic sanity check by pip installing the module, and then

• check >>> import ligo.gracedb
• check gracedb --help