re-added GCC6

.travis.yml

@@ -61,13 +61,13 @@ matrix:
        - mkdir $HOME/clang-$LLVM_VERSION
        - tar xf $LLVM_ARCHIVE_PATH -C $HOME/clang-$LLVM_VERSION --strip-components 1
 
-#    - os: linux
-#      compiler: gcc
-#      addons:
-#        apt:
-#          sources: ['ubuntu-toolchain-r-test']
-#          packages: ['g++-6', 'valgrind']
-#      env: COMPILER=g++-6
+    - os: linux
+      compiler: gcc
+      addons:
+        apt:
+          sources: ['ubuntu-toolchain-r-test']
+          packages: ['g++-6', 'valgrind']
+      env: COMPILER=g++-6
 
     # Clang 3.5 is not able to compile the code,
     # see https://travis-ci.org/nlohmann/json/jobs/126720186