Add TruffleRuby in CI

Author: eregon

.github/workflows/ci.yml

@@ -45,6 +45,7 @@ jobs:
           - "3.0.0"
           - ruby-head
           - jruby-9.2
+          - truffleruby-head
     steps:
       - uses: actions/checkout@v1
       - name: Set up Ruby