Disable testing against ruby head for now

Author: alexdalitz

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.5', '2.6', '2.7', '3.0', 'head' ]
+        ruby: [ '2.5', '2.6', '2.7', '3.0' ]
     name: Ruby ${{ matrix.ruby }} tests
     steps:
       - uses: actions/checkout@v2