Fix backwards compat issue with t.w.client.Agent._contextFactory.

Make Twisted's changes for their implementation of the SSL (non-strict)
hostname verification (in Twisted-14.1.0) backwards compatible with ours
(and Twisted-13.2.0).

In Twisted-14.1.0, `twisted.web.client.Agent._contextFactory`, which
contained the original OpenSSL context factory for the connection, was
renamed to `twisted.web.client.Agent._policyForHTTPS`. We access
`_contextFactory` in our unittests to test our implementation of a
strict SSL hostname verification context
(`bridgedb.crypto.SSLVerifyingContextFactory`).

 * FIXES a backwards compatibility issue introduced by Twisted's
   implementation of SSL hostname verification
   (https://twistedmatrix.com/trac/ticket/4888) in commit
   https://twistedmatrix.com/trac/changeset/42546.