Use a more recent ubuntu version.

jschlumpp · jschlumpp · commit 6140a89ce823 · 2017-07-17T23:00:55.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,5 @@
 sudo: required
+dist: trusty
 language: rust
 rust:
   - nightly
