Add ruby 3 (#129)

Author: ferhatelmas

.github/workflows/ci.yml

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