Travis CI: Run `make xref` in addition to `make tests`

While here, test on Erlang 19.3 (instead of 19.0) and Erlang 20.1. Also,
we stop testing on Erlang 17.5 and 18.3: it helps reduce the number of
jobs in Travis CI and allow it to go over all changes quicker.

Finally, the sections are reordered so the scripts are at the end. It
may help in the future if we want to template all the `.travis.tml`
files.

[#152509619]

deps/rabbit_common/.travis.yml

@@ -1,3 +1,5 @@
+# vim:sw=2:et:
+
 sudo: false
 language: erlang
 notifications:
@@ -7,26 +9,27 @@ addons:
   apt:
     packages:
       - xsltproc
+cache:
+  apt: true
+
 otp_release:
   - "R16B03-1"
-  - "17.5"
-  - "18.3"
-  - "19.0"
+  - "19.3"
+  - "20.1"
 
-# The checkout made by Travis is a "detached HEAD" and branches
-# information is missing. Our Erlang.mk's git_rmq fetch method relies on
-# it, so we need to restore it.
-#
-# We simply fetch master and, if it exists, stable branches. A branch is
-# created, pointing to the detached HEAD.
 before_script:
+  # The checkout made by Travis is a "detached HEAD" and branches
+  # information is missing. Our Erlang.mk's git_rmq fetch method relies
+  # on it, so we need to restore it.
+  #
+  # We simply fetch master and, if it exists, stable branches. A branch
+  # is created, pointing to the detached HEAD.
   - |
     git checkout -B "${TRAVIS_TAG:-${TRAVIS_BRANCH}}"
     git remote add upstream https://github.com/$TRAVIS_REPO_SLUG.git
     git fetch upstream stable:stable || :
     git fetch upstream master:master || :
 
-script: make tests
-
-cache:
-  apt: true
+script:
+  - make xref
+  - make tests