CI: Use Ubuntu Xenial

zeenix · zeenix · commit 0b6e43be5bdb · 2019-06-18T17:51:15.000+02:00
Trusty is now way too old and you can't even find package names for it on
packages.ubuntu.com.
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,4 @@
-dist:     trusty
+dist:     xenial
 sudo:     false
 language: go
 go:
