Include Ruby 2.7 in test matrix

Author: vcsjones

.github/workflows/ruby.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.6', '3.0' ]
+        ruby: [ '2.6', '2.7', '3.0' ]
 
     steps:
     - uses: actions/checkout@master