Add webhook to invoke a Diffblue-specific function

Triggers compatibility checks for the current PR.

The plan is to then give feedback on the PR as to whether it is
compatible with Diffblue products, but this part is not yet merged.

Function will be moved to the Diffblue cloud (instead of joelallred)
once the framework is finalised.

.travis.yml

@@ -286,6 +286,7 @@ notifications:
   webhooks:
     urls:
       - http://dashboard.diffblue.com/api/travis-webhooks
+      - https://us-central1-dev-user-joelallred.cloudfunctions.net/trigger-testgen-from-cbmc
     on_success: always
     on_failure: always
     on_start: never