set gcc in travis

.travis.yml

 language: python
+matrix:
+  include:
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.9
+      env:
+         - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
+
+    # works on Precise and Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-5
+      env:
+         - MATRIX_EVAL="CC=gcc-5 && CXX=g++-5"
 python:
   - "2.7"
   - "3.6"
 # command to install dependencies
+before_install:
+    - eval "${MATRIX_EVAL}"
 install:
   - pip install --upgrade pip
   - pip install -r requirements.txt