Add Ruby v2.7 to CI build (#52)

Author: sausman

.github/workflows/ruby.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       matrix:
         os: ["ubuntu-latest"]
-        ruby: ["2.3", "2.4", "2.5", "2.6"]
+        ruby: ["2.3", "2.4", "2.5", "2.6", "2.7"]
 
     runs-on: ${{ matrix.os }}