Don't use verify_callback.

The OpenSSL verify_callback isn't supported on jruby [1], and behaves in
somewhat surprising ways on OS X due to Apple monkey patching OpenSSL.

We probably want to move in the direction of just passing through the
OpenSSL exceptions anyway.

[1] https://github.com/jruby/jruby/issues/597

Fixes: #165
See also: #168, e03e5e6c