testing: force travis to use trusty distro

This matches what libc does

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

.travis.yml

@@ -1,10 +1,9 @@
----
+language: rust
 sudo: required
+dist: trusty
 services:
   - docker
 
-# language: rust
-
 # rust:
 #   - 1.1.0  # Oldest supported version
 #   - 1.2.0