Replace Travis CI references with Gitlab CI.

[RobertImbrie]

This PR closes #677.

• Change "Travis CI" to "Gitlab CI" in CONTRIBUTING.md.
• Change ".travis.yaml" to ".gitlab-ci.yaml" in DistributeTests.ps1.

Note: In the issue (and in other parts of the repository), it's spelled "Gitlab," which I defaulted to in order to keep things consistent. However, it's officially spelled "GitLab," with a capitalized L. We should open another issue to fix this.

[desilinguist]

Thanks for this @RobertImbrie!

Turns out that Gitlab CI for Github will not build PRs from external forks 🤦🏽(though they have been debating it for almost 2 years now!) Since this is our first external PR via a fork after migrating to Gitlab CI, I wasn't aware of this until now.

For PRs with non-code changes like this one, we can just rely on the Azure Pipelines build and merge without the Gitlab CI check. However, for PRs with code changes, we will need to figure out a workaround. Here's one possible solution:

• When a new external PR comes in, create a new local branch and ask the author of the PR to change the target of their PR to that branch instead of main.
• Then we merge that PR and open a new PR from the local branch to main.
• Gitlab CI will then run just fine.

Thoughts @aoifecahill @mulhod @bndgyawali ? Of course, ideally, we should automate this process using a bot or something like GitHub Actions?

[aoifecahill]

Yeah, that process makes sense to me. And I agree, automating it would be ideal.