Move to Gitlab CI

This implements the CI as we had it in Github, except it only uses Gitlab facilities now:

• Gitlab CI instead of Travis CI
• the Gitlab registry instead of the Docker Hub

In addition, we gain two nifty features:

• the built tarball is available for download as a CI artifact;
• the libcangjie Docker image is only built after merging into master, with all build configuration in Gitlab rather than in the Docker Hub;

This depends on build-essential!7 (merged)