Adding gcc 5 and 6

.travis.yml

@@ -7,6 +7,22 @@ cache:
 
 matrix:
     include:
+        - compiler: gcc
+          addons:
+              apt:
+                  sources:
+                      - ubuntu-toolchain-r-test
+                  packages:
+                      - g++-6
+          env: COMPILER=6
+        - compiler: gcc
+          addons:
+              apt:
+                  sources:
+                      - ubuntu-toolchain-r-test
+                  packages:
+                      - g++-5
+          env: COMPILER=5
         - compiler: gcc
           addons:
               apt: