Travis CI: Update config from rabbitmq-common

deps/rabbitmq_auth_backend_cache/.travis.yml

@@ -31,11 +31,20 @@ otp_release:
   - '22.2'
 
 install:
-  - mix local.hex --force
+  # This project being an Erlang one (we just set language to Elixir
+  # to ensure it is installed), we don't want Travis to run mix(1)
+  # automatically as it will break.
+  skip
 
 script:
+  # $current_rmq_ref is also used by rabbitmq-components.mk to select
+  # the appropriate branch for dependencies.
+  - make check-rabbitmq-components.mk
+    current_rmq_ref="${TRAVIS_PULL_REQUEST_BRANCH:-${TRAVIS_BRANCH}}"
   - make xref
+    current_rmq_ref="${TRAVIS_PULL_REQUEST_BRANCH:-${TRAVIS_BRANCH}}"
   - make tests
+    current_rmq_ref="${TRAVIS_PULL_REQUEST_BRANCH:-${TRAVIS_BRANCH}}"
 
 after_failure:
   - |