travis: use Erlang v23

Remove branch builds.

Signed-off-by: Matteo Cafasso <[email protected]>

Author: noxdafox

.travis.yml

@@ -2,13 +2,11 @@ dist: bionic
 language: elixir
 elixir: '1.10.4'
 otp_release:
-  - '22.3'
+  - '23.0.4'
 install:
   - wget https://github.com/rabbitmq/mix_task_archive_deps/releases/download/0.5.0/mix_task_archive_deps-0.5.0.ez
   - mix archive.install --force ./mix_task_archive_deps-0.5.0.ez
 script: make tests current_rmq_ref=$TRAVIS_BRANCH
 branches:
   only:
     - master
-    - v3.7.x
-    - v3.8.x