downgrade otp as 23.1 is not yet available on travis

Author: lukaszsamson

.travis.yml

@@ -32,7 +32,7 @@ matrix:
       elixir: 1.9.4
     - otp_release: 23.0.4
       elixir: 1.10.4
-    - otp_release: 23.1.1
+    - otp_release: 23.0.4
       elixir: 1.11.1
       env:
         - CHECK_FORMATTED=1