Add Ruby 3.2 to GitHub CI file (#445)

* Also removes Travis CI file because it seems like this project
  has moved to GitHub CI.

Author: jessieay

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', truffleruby-head]
+        ruby-version: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', truffleruby-head]
 
     steps:
     - uses: actions/checkout@v2