Pin flake8 to specific version on travis (#1242)

Author: sbc100

.travis.yml

@@ -19,7 +19,7 @@ jobs:
         - export CXX="${CXX_COMPILER}"
         - export ASAN_OPTIONS="symbolize=1"
       install:
-        - pip install --user flake8
+        - pip install --user flake8==3.4.1
       before_script:
         # Check the style of a subset of Python code until the other code is updated.
         - flake8 ./scripts/

appveyor.yml

@@ -21,7 +21,7 @@ environment:
     PARALLEL_FLAG: "/m:"
 
 install:
-  - pip install flake8
+  - pip install flake8==3.4.1
 
 before_build:
   # Check the style of a subset of Python code until the other code is updated.