Only run flake8 on one of the CI builds

.travis.yml

@@ -1,6 +1,3 @@
-dist: trusty
-group: beta
-sudo: false
 addons:
   apt:
     packages:
@@ -14,6 +11,22 @@ addons:
 language: python
 python:
   - "3.5"
+matrix:
+    include:
+        - os: linux
+          dist: trusty
+          group: beta
+          sudo: false
+          env:
+            - CC=gcc ASANLIB=libasan.so.0 ASAN_ARG=--asan
+        - os: linux
+          dist: trusty
+          group: beta
+          sudo: false
+          env:
+            - CC=clang
+            - RUN_FLAKE=1
+
 install:
     - pip install flake8
     - wget -O glfw-3.2.1.zip https://github.com/glfw/glfw/archive/3.2.1.zip
@@ -29,11 +42,8 @@ env:
         - PKG_CONFIG_PATH=$HOME/glfw/lib/pkgconfig 
         - LD_LIBRARY_PATH=$HOME/glfw/lib 
         - ASAN_OPTIONS=leak_check_at_exit=0 
-    matrix:
-        - CC=gcc ASANLIB=libasan.so.0 ASAN_ARG=--asan
-        - CC=clang
 before_script:
     - python setup.py build --debug $ASAN_ARG
 script: 
     - LD_PRELOAD=$ASANLIB python setup.py test
-    - flake8 .
+    - if [[ "$RUN_FLAKE" == "1" ]]; then flake8 .; fi