Roll back pyOpenSSL requirement.

There are some unresolved bugs in requests' pyOpenSSL backend which are
causing too many errors right now. We should definitely use the
pyOpenSSL backend eventually for SNI support and other improvements, but
right now it's causing too much noise while trying to shake out other
bugs via stress testing. We'll revisit this decision before beta launch.

There is ongoing work on this upstream which will hopefully be resolved
in the next few weeks or months:

(See https://github.com/shazow/urllib3/pull/371