Change GitHub CI to install Ruby 2.2 on the latest Ubuntu

Author: piotrmurach

.github/workflows/ci.yml

@@ -23,6 +23,7 @@ jobs:
         ruby:
           - "2.0"
           - "2.1"
+          - "2.2"
           - "2.3"
           - "2.4"
           - "2.5"
@@ -37,8 +38,6 @@ jobs:
           - jruby-head
           - truffleruby-head
         include:
-          - ruby: "2.2"
-            os: ubuntu-20.04
           - ruby: "2.7"
             coverage: true
           - ruby: jruby-9.2