Add truffleruby-head to CI matrix

This supersedes redis#70.

Author: stanhu

.github/workflows/test.yaml

@@ -7,7 +7,7 @@ jobs:
       fail-fast: false
       matrix:
         os: ["ubuntu-latest"]
-        ruby: [2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, jruby]
+        ruby: [2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, jruby, trufflehead-ruby]
     runs-on: ${{ matrix.os }}
     services:
       redis: