remove test support for ruby 2.5

Author: fedetaglia

.github/workflows/build.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: ['2.5', '2.6', '2.7']
+        ruby: ['2.6', '2.7']
     name: Test ruby v${{ matrix.ruby }} support
     steps:
     - uses: actions/checkout@v1