Add all remaining ruby 2.x versions to build matrix

These builds now all pass for the time being, so since they were in the
original .travis.yml I'm adding them here.

To get ruby 2.2 working with the official setup-ruby action I've had to
pin the runner on ubuntu-20.04 due to a known issue with setup-ruby, 2.2
and ubuntu-latest: ruby/setup-ruby#496

.github/workflows/ci.yaml

@@ -11,11 +11,16 @@ permissions:
 jobs:
   rspec:
     name: RSpec (ruby ${{ matrix.ruby-version }})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
         ruby-version:
+          - '2.0'
+          - '2.1'
+          - '2.2'
+          - '2.3'
+          - '2.4'
           - '2.5'
           - '2.6'
           - '2.7'