Make the gcc6/python 3.5 build use debug mode

.travis.yml

@@ -19,7 +19,7 @@ matrix:
     env: PYTHON=2.7 CPP=14 GCC=6
   - sudo: true
     services: docker
-    env: PYTHON=3.5 CPP=14 GCC=6
+    env: PYTHON=3.5 CPP=14 GCC=6 DEBUG=1
   - os: osx
     osx_image: xcode7.3
     env: PYTHON=2.7 CPP=14 CLANG
@@ -59,6 +59,7 @@ before_install:
   fi
   if [ -n "$CPP" ]; then export CPP=-std=c++$CPP; fi
   if [ "${PYTHON:0:1}" = "3" ]; then export PY=3; fi
+  if [ -n "$DEBUG" ]; then export CMAKE_EXTRA_ARGS="-DCMAKE_BUILD_TYPE=Debug"; fi
 - |
   # Initialize enviornment
   if [ -n "$DOCKER" ]; then