Update .travis.yml

Author: siscia

.travis.yml

@@ -34,6 +34,7 @@ matrix:
     - env: TARGET=powerpc64le-unknown-linux-gnu
     - env: TARGET=s390x-unknown-linux-gnu DISABLE_TESTS=1
     - env: TARGET=x86_64-unknown-linux-gnu
+      dist: trusty
     - env: TARGET=x86_64-unknown-linux-musl
 
     # OSX