Pin versions of used pip packages

To avoid breaking our builds because of changes in pip packages we use
for testing it makes sense to introduce version pinning to the travis
build. (see #287, #295)

Author: behrtam

.travis.yml

@@ -9,7 +9,9 @@ python:
   - 3.5
 
 install:
-  - pip install flake8
+  - pip install pep8==1.7.0
+  - pip install pyflakes==1.0.0
+  - pip install flake8==2.5.2
 
 before_script:
   - flake8 ./exercises/ --max-line-length=99 --select=E,W

requirements-travis.txt

@@ -0,0 +1,3 @@
+flake8==2.5.2
+pep8==1.7.0
+pyflakes==1.0.0