Cleanup Continuous Integration (#384)

* Remove "stale" workflow. It was a nice idea, but deactivated for quite some time anyway

* Remove TravisCI

.github/workflows/stale.yml

@@ -1,19 +0,0 @@
-name: "Close stale issues"
-on:
-  schedule:
-  - cron: "0 4 * * *"
-
-jobs:
-  stale:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/stale@v1.1.0
-      with:
-        repo-token: ${{ secrets.GITHUB_TOKEN }}
-        stale-issue-message: >
-          This issue has been automatically marked as stale because it has not had
-          recent activity in the last 60 days. It will be closed in 7 days if no further activity occurs.
-          Thank you for your contributions.
-        days-before-stale: 9999
-        days-before-close: 9999
-        stale-issue-label: stale