Travis CI: Run make xref in addition to make tests

While here, test on Erlang 20.1. Also, we stop testing on Erlang 19.2:
it's now unsupported.

We take Elixir from Erlang Solutions packages because kiex is affected
by GitHub API rate limiting.

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]

Author: dumbbell

.travis.yml

@@ -1,16 +1,26 @@
 # vim:sw=2:et:
-sudo: false
 
+sudo: false
 language: erlang
-
 notifications:
   email:
     - [email protected]
+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:
+      # 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
 
 otp_release:
-  - "19.2"
   - "19.3"
-  - "20.0"
+  - "20.1"
 
 before_script:
   # The checkout made by Travis is a "detached HEAD" and branches
@@ -24,12 +34,12 @@ before_script:
     git remote add upstream https://github.com/$TRAVIS_REPO_SLUG.git
     git fetch upstream stable:stable || :
     git fetch upstream master:master || :
-  - kiex selfupdate
-  - test -x ~/.kiex/elixirs/elixir-1.4.5/bin/elixir || kiex install 1.4.5
-  - kiex default 1.4.5
-
-script: make tests
+  # 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'
 
-cache:
-  directories:
-    - ~/.kiex/elixirs
+script:
+  - make xref
+  - make tests