Migrate CI builds to GitLab CI

[desilinguist]

Since Travis CI no longer seems to be offering open source credits for free, we need to move to GitLab CI since it works with GitHub repos.

[desilinguist]

See also #494.

[desilinguist]

Here is the process to follow for the main RSMTool repository:

1. Follow the steps here to set up a mirror repository on Gitlab only for CI/CD purposes.

2. Set up a new Slack webhook to post to the #rsmtool-dev channel as "Gitlab CI". Copy the webhook URL.

3. Go to the newly mirrored project on GitLab. Go to Settings > Integration. Choose "Slack notifications". Uncheck all checkboxes except "Enable Integration - Active" and "Pipeline". Paste the Slack webhook URL in the "Webhook" field. Uncheck "Notify only broken pipelines" and change "Branches to be notified" dropdown to "All Branches". Click the "Save Changes" button.

4. In this Github repository, go to Settings > Branches > Branch Protection Rule. Edit the rule for the main branch. Look in the Status Checks section, uncheck "Travis CI - Branch" and "Travis CI - Pull Request" and hit the Save button at the bottom.

5. Create a new RSMTool branch with the same changes as shown in this SKLL PR. DO NOT SUBMIT A PULL REQUEST YET.

6. Once the Gitlab CI builds pass for this new branch, go back to the status checks in Step 1. You should now see a "ci/gitlab/gitlab.com" item. Enable that so that it becomes a required check and then click the Save button again.

7. Now submit a PR with this new branch.

[desilinguist]

And, of course, the builds for rsmtool-conda-tester and rsmtool-pip-tester also need to be moved to Gitlab CI. First, mirror these two repos to Gitlab and add Slack notifications just like the main repository above. Use "rsmtool-conda-linux64" and "rsmtool-pip-linux64" as the names of the Slack bots. Then submit PRs based on the corresponding SKLL PRs here and here.

[desilinguist]

@Frost45 here's the issue that explains how to migrate the 3 rsmtool repositories to Gitlab CI.