Fix link to build matrix in COMPATIBILITY.md

We switched from Travis to CircleCI, so this needs to point at a
different file now.

Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

COMPATIBILITY.md

@@ -8,7 +8,7 @@ Any Ruby version that has not received an End-of-Life notice (e.g.
 is supported.
 
 To ensure we're meeting these guidelines, we test the client against all
-supported versions, as specified in our [build matrix](.travis.yml).
+supported versions, as specified in our [build matrix](.circleci/config.yml).
 
 # Deprecation