Travis CI: Update config from rabbitmq-common

deps/rabbitmq_auth_backend_cache/.travis.yml

@@ -7,9 +7,15 @@ notifications:
     - alerts@rabbitmq.com
 addons:
   apt:
+    sources:
+      - sourceline: deb https://packages.erlang-solutions.com/ubuntu trusty contrib
+        key_url: https://packages.erlang-solutions.com/ubuntu/erlang_solutions.asc
     packages:
       - awscli
-      - xsltproc
+      # Use Elixir from Erlang Solutions. The provided Elixir is
+      # installed with kiex but is old. We also can't use kiex to
+      # install a newer one because of GitHub API rate limiting.
+      - elixir=1.4.5-1
 cache:
   apt: true
 env:
@@ -18,7 +24,6 @@ env:
     - secure: 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
 
 otp_release:
-  - "R16B03-1"
   - "19.3"
   - "20.1"
 
@@ -34,6 +39,11 @@ before_script:
     git remote add upstream https://github.com/$TRAVIS_REPO_SLUG.git
     git fetch upstream stable:stable || :
     git fetch upstream master:master || :
+  # Make sure we use Elixir from Erlang Solutions and not kiex.
+  - |
+    echo YES | kiex implode
+    elixir --version
+    elixir --version | grep -q 'Elixir 1.4.5'
 
 script:
   - make xref