TST: Use benchmark returns from spreadsheet.

The risk unit tests were using the public Yahoo! data instead
of the returns from the answer key spreadsheet, change the RiskPeriod's
created in tests to use the values in the benchmark returns
column of the answer key.

Also, change the spreadsheet's benchmark volatility calculation
to use sample.
The use of population was exposed when the input values were
corrected.
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.
ENH: Add OHLC to simulated data source
BLD: Fix import of answer key for compatibility with Python 3.

Python 3 requires explicit relative pathing.
Add execution styles to zipline.