ci: run CI for Racket 6.6 and up

Racket 6.5 has an SSL issue that makes it unable to install packages.
That might be why it was not included in the Travis build either.

.github/workflows/ci.yml

@@ -9,8 +9,23 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        racket-version: ["7.4", "7.5", "7.6", "7.7", "7.8", "7.9", "current"]
+        racket-version: ["6.6", "6.7", "6.8", "6.9", "6.10.1", "6.11", "6.12", "7.0", "7.1", "7.2", "7.3", "7.4", "7.5", "7.6", "7.7", "7.8", "7.9", "current"]
         racket-variant: ["BC", "CS"]
+        # CS builds are only provided for versions 7.4 and up so avoid
+        # running the job for prior versions.
+        exclude:
+          - {racket-version: "6.6",    racket-variant: "CS"}
+          - {racket-version: "6.7",    racket-variant: "CS"}
+          - {racket-version: "6.8",    racket-variant: "CS"}
+          - {racket-version: "6.9",    racket-variant: "CS"}
+          - {racket-version: "6.10.1", racket-variant: "CS"}
+          - {racket-version: "6.11",   racket-variant: "CS"}
+          - {racket-version: "6.12",   racket-variant: "CS"}
+          - {racket-version: "7.0",    racket-variant: "CS"}
+          - {racket-version: "7.1",    racket-variant: "CS"}
+          - {racket-version: "7.2",    racket-variant: "CS"}
+          - {racket-version: "7.3",    racket-variant: "CS"}
+
     steps:
     - name: Checkout
       uses: actions/checkout@master