Build on Ubuntu 16.04 instead of 14.04

.travis.yml

@@ -1,7 +1,7 @@
 # Based on the "trust" template v0.1.2
 # https://github.com/japaric/trust/tree/v0.1.2
 
-dist: trusty
+dist: xenial
 language: rust
 services: docker
 sudo: required
@@ -16,14 +16,8 @@ matrix:
   # Linux
   - env: TARGET=arm-unknown-linux-gnueabi
     rust: nightly
-  - env: TARGET=x86_64-unknown-linux-gnu CC=gcc-7
+  - env: TARGET=x86_64-unknown-linux-gnu
     rust: nightly
-    addons:
-      apt:
-        packages:
-          - gcc-7
-        sources:
-          - ubuntu-toolchain-r-test
   - env: TARGET=x86_64-unknown-linux-musl
     rust: nightly