Add GCC 4.9 to travis tests

Author: gummif

.travis.yml

@@ -60,6 +60,17 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
 
+    # GCC 4.9, draft disabled (default), latest libzmq (default)
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.9
+      env:
+        - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
+
     # GCC 7, draft enabled, latest libzmq (default)
     - os: linux
       addons: