coursier: add shell completions

[darklore]

• [x] Have you followed the guidelines for contributing?
• [x] Have you ensured that your commits follow the commit style guide?
• [x] Have you checked that there aren't other open pull requests for the same formula update/change?
• [x] Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [x] Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• [x] Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

---

[darklore]

Sorry, I forgot incrementing revision number. I added a commit to do it.

[BrewTestBot]

:robot: A scheduled task has triggered a merge.