Add gcc-7 to Travis

.travis.yml

@@ -117,6 +117,16 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
+    - os: linux
+      compiler: g++-7
+      env: TOOLSET=gcc COMPILER=g++-7 CXXSTD=c++11
+      addons:
+        apt:
+          packages:
+            - g++-7
+          sources:
+            - ubuntu-toolchain-r-test
+
     - os: linux
       compiler: clang-3.5
       env: TOOLSET=clang COMPILER=clang++-3.5 CXXSTD=c++11