Added back support for GCC 5.

.travis.yml

@@ -14,6 +14,16 @@ addons:
       - re2c
 matrix:
   include:
+    - env: COMPILER_NAME=g++ _CXX=g++-5 _CC=gcc-5
+      os: linux
+      language: cpp
+      addons:
+        apt:
+          sources:
+            - *default_sources
+          packages:
+            - *default_packages
+            - g++-5
     - env: COMPILER_NAME=g++ _CXX=g++-6 _CC=gcc-6
       os: linux
       language: cpp