ci: be less precise with our ruby version with setup-ruby

We constantly are fighting updating these minor versions because rbenv wants us
to be precise. But the ruby-setup action we rely on for the Ubuntu builds
has a much more restricted list of versions which are available.

Keeping both as the same list means CI breaks constantly for no reason so create
two different lists to try to stop us having to do busywork.

Author: carlosmn

.github/workflows/ci.yml

@@ -12,12 +12,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.4.9', '2.5.7', '2.6.5', '2.7.0' ]
-        os: [ ubuntu-18.04, macOS-10.14 ]
+        ruby-mac: [ '2.4.9', '2.5.7', '2.6.5', '2.7.0' ]
+        ruby-linux: [ '2.4', '2.5', '2.6', '2.7' ]
+        os: [ ubuntu-18.04, macOS-latest ]
 
     runs-on: ${{ matrix.os }}
 
-    name: Ruby ${{ matrix.ruby }} on ${{ matrix.os }}
+    name: Ruby ${{ matrix.ruby-mac }} ${{ matrix.ruby-linux }} on ${{ matrix.os }}
     steps:
     - uses: actions/checkout@master
     - name: update submodule
@@ -34,13 +35,13 @@ jobs:
       if: runner.os == 'Linux'
       uses: actions/setup-ruby@v1
       with:
-        ruby-version: ${{ matrix.ruby }}
+        ruby-version: ${{ matrix.ruby-linux }}
     - name: Set up Ruby on macOS
       if: runner.os == 'macOS'
       run: |
         brew install rbenv
-        rbenv install ${{ matrix.ruby }}
-        rbenv local ${{ matrix.ruby }}
+        rbenv install ${{ matrix.ruby-mac }}
+        rbenv local ${{ matrix.ruby-mac }}
     - name: run build
       run: |
         if [ -x rbenv ]; then eval "$(rbenv init -)"; fi