Travis CI: Use Ubuntu Trusty image to have access to Elixir

Also, required `sudo` so a VM is used instead of a container. This makes
the test much longer, but allows us to install non-whitelisted packages
such as xsltproc.

deps/rabbitmq_tracing/.travis.yml

@@ -1,4 +1,10 @@
-sudo: false
+# vim:sw=2:et:
+
+# Use Ubuntu Trusty to have access to Elixir.
+dist: trusty
+group: beta
+sudo: required
+
 language: erlang
 notifications:
   email: