Add 2.x.x-stable branch to Travis CI

(cherry picked from commit f00e7b1)

.travis.yml

@@ -18,17 +18,18 @@ addons:
       - sourceline: deb https://dl.bintray.com/rabbitmq/debian trusty main
         key_url: https://dl.bintray.com/rabbitmq/Keys/rabbitmq-release-signing-key.asc
     packages:
-      - esl-erlang=1:20.2.2
+      - esl-erlang=1:20.3
 # because of https://github.com/travis-ci/travis-ci/issues/8906
 before_install:
   - sudo mv /opt/jdk_switcher/jdk_switcher.sh /tmp
-  - sudo apt-get install rabbitmq-server=3.7.4-1
+  - sudo apt-get install rabbitmq-server=3.7.7-1
   - sudo mv /tmp/jdk_switcher.sh /opt/jdk_switcher/
 services:
   - rabbitmq
 branches:
   only:
     - master
+    - 2.x.x-stable
     - 1.3.x-stable
 before_script:
   - ./bin/before_build.sh