CI fixes + Delete empty .gitmodules

[Croydon]

DON'T MERGE YET

This will be a control pull request after #468 is merged into master.

Let's see if everything works as expected 😄

Edit:

• this should fix the tagging in the master branch with I broke with #468
• add Clang 5 to your CI (fix #505)
  • I love the issue number in this context 😁