travis: patch pyflakes for devel Python.

Fixes:

    AttributeError: 'Checker' object has no attribute 'CONSTANT'

PyCQA/pyflakes#367

.travis.yml

@@ -30,6 +30,9 @@ before_install:
 install:
 - pip install "futures; python_version < '3'"
 - pip install $pyflakes
+- if [[ $TRAVIS_PYTHON_VERSION = *-dev ]]; then
+    printf '\n\n    CONSTANT = ignore\n' >> $VIRTUAL_ENV/lib/python*/site-packages/pyflakes/checker.py;
+  fi
 - $if_pylint pip install pylint
 - pip install docutils
 script: