Gitlab CI config

Created by: alatiera

Preparing for the gitlab migration

One thing to note is that travis was using a ubuntu trusty base, where now we are using the rust docker images, both based on debian stable. Sadly the tarball hack is still with us, though I plan on getting rid of it in the future.

There is also a rustfmt job, currently the pipeline becomes red if it fails, but it can also be set to allow_failure: true.

Pipeline results: https://gitlab.freedesktop.org/alatiera/gstreamer-rs/pipelines/5458