Fix typos in ssladapter.py

- Check that `urllib_ver` is not None, instead of `urllib3`
- Do version comparison only if `urllib_ver` is *not* "dev"

Fixes #325. Original work by @sidprak.
Enable the overriding of `assert_hostname` on TLSConfig

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
Merge pull request #347 from aanand/override-assert-hostname

Enable the overriding of `assert_hostname` on TLSConfig