Tweak workflow labelling and triggers

- Rename the workflow from "Ruby" to "CI" to avoid redundant prefix

- Append Gemfile from matrix to job name, so they are distinguishable

.github/workflows/ruby.yml → .github/workflows/ci.yml

@@ -1,12 +1,12 @@
-name: Ruby
+name: CI
 
 on: [push, pull_request]
 
 jobs:
   build:
 
     runs-on: ubuntu-latest
-    name: Ruby ${{ matrix.ruby }}
+    name: Ruby ${{ matrix.ruby }} / ${{ matrix.gemfile }}
     strategy:
       matrix:
         gemfile: [Gemfile, gemfiles/rails_6_0.gemfile, gemfiles/rails_edge.gemfile]