travis: fix branch build

RabbitMQ Makefile automatically detects the dependencies version from
the branch name. Travis checkouts to the head of the branch loosing
such information.

Therefore, we need to provide the branch information in some other way.

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

Author: noxdafox

.travis.yml

@@ -3,7 +3,7 @@ language: elixir
 elixir: '1.6.6'
 otp_release:
   - '21.0'
-script: make tests
+script: make tests current_rmq_ref=$TRAVIS_BRANCH
 branches:
   only:
     - master