Test GCC 5.5

.travis.yml

@@ -6,6 +6,14 @@ sudo: false
 
 matrix:
   include:
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+             - ubuntu-toolchain-r-test
+          packages:
+             - g++-5
+      env: COMPILER=g++-5 CXXSTD=11  
     - compiler: gcc
       addons:
         apt:
@@ -128,6 +136,14 @@ matrix:
              - libc++-dev
       env: COMPILER=clang++-6.0 CXXSTD=11
 
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+             - ubuntu-toolchain-r-test
+          packages:
+             - g++-5
+      env: COMPILER=g++-5 CXXSTD=14
     - compiler: gcc
       addons:
         apt: