force Ubuntu xenial in travis

Speedy37 · Speedy37 · commit 1e381efceae3 · 2019-06-25T23:24:31.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -12,6 +12,8 @@ os:
   - linux
   - osx
 
+dist: xenial
+
 matrix:
   allow_failures:
     - rust: nightly
