Travis should use gcc-7 on linux

.travis.yml

@@ -4,11 +4,20 @@ matrix:
       dist: trusty
       sudo: required
       compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+      env:
+        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
     - os: osx
       osx_image: xcode7.2
       compiler: clang
 
 before_install:
+  - eval "${MATRIX_EVAL}"
   - |
     # OSX
     if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then