Tweak travis: test both trusty and xenial

Author: jeroen

.travis.yml

@@ -7,22 +7,23 @@ r_check_args: '--run-donttest'
 
 matrix:
   include:
-    - os: linux
-      addons:
-        apt:
-          packages: cargo
+    - dist: trusty
+    - dist: xenial
     - os: linux
       addons:
         apt:
           sources:
             sourceline: 'ppa:ubuntu-mozilla-security/rust-updates'
           packages: cargo
     - os: osx
-      osx_image: xcode9.3
       brew_packages: rust
     - os: osx
       osx_image: xcode8.3
       disable_homebrew: true
     - os: osx
       osx_image: xcode7.3
       disable_homebrew: true
+
+addons:
+  apt:
+    packages: cargo