BLD: Peg CI versions of lint checker libraries.

Match the revisions found in the dev requirements file, so that the
versions used during development and continuous integration.

Pegging the versions in two separate places may end being brittle,
but suppress the build failure because the unpegged installation
of flake8 pulled in newer versions of pyflakes and pep8.

.travis.yml

@@ -12,7 +12,7 @@ install:
   - conda create -n testenv --yes pip python=$TRAVIS_PYTHON_VERSION
   - source activate testenv
   - conda install --yes -c https://conda.binstar.org/twiecki numpy==1.8.0 scipy nose matplotlib pandas Cython patsy statsmodels tornado pyparsing xlrd mock pytz requests six dateutil ta-lib logbook
-  - pip install flake8 nose-parameterized==0.3.3 nose-ignore-docstring==0.2
+  - pip install pyflakes==0.7.3 pep8==1.4.6 mccabe==0.2.1 flake8==2.1.0 nose-parameterized==0.3.3 nose-ignore-docstring==0.2
 before_script:
   - "flake8 zipline tests"
 script: