include gcc7 in tests

Author: Simon Pickartz

.travis.yml

@@ -34,6 +34,16 @@ matrix:
       env:
           - COMPILERCXX=g++-6
           - COMPILERC=gcc-6
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+      env:
+          - COMPILERCXX=g++-7
+          - COMPILERC=gcc-7
     - compiler: clang
       env:
           - COMPILERC=clang