Remove incompatible SSL option in Python 2.6

An exposed SSL option, `ciphers`, was added in Python 2.7 and breaks older
versions of Python with an unexpected kwargs error.

I'm not sure if this is the best way to fix this, and I'm happy to consider
other approachs. Some things I'm a bit up in the air about are:

  i) This is only broken for Python 2.6 and older whose ssl module doesn't
  expose this. Do the other types of workers not use ssl module, and thus might
  be able to make use of the `cipher` kwarg?

  ii) Should we silently or explicity fail? I couldn't find a nice way to do
  this because the configuration setting has a default value and resulted in
  stringy code to special case the one setting`.

What I would really like to do would add a "python_version" validator that
could intelligently handle this, but I went off into the weeds figuring out
how the SettingsMeta class works. :)
Define `Ciphers` config conditionally

We only expose the `Ciphers` config parameter in versions of Python that
support it (i.e., >= Python 2.7).
Merge pull request #725 from sholsapp/master

Remove incompatible SSL option in Python 2.6