ci: switch to xenial

Rust is having problems with trusty, in particular, see this bug I
filed: rust-lang/rust#59411

This was purpotedly fixed in
rust-lang/rust#59468,
but it appears the issue is still occurring.

This commit tries to update to Ubuntu 16.04 in the hope that it will fix
this problem.

.travis.yml

@@ -1,4 +1,5 @@
 language: rust
+dist: xenial
 env:
   global:
     - PROJECT_NAME: ripgrep