Allow use of TLS

Resolves #17.

- Set `ssl_version` argument to `ssl.PROTOCOL_SSLv23` on `Connection` from
  stomp.py so that the highest SSL/TLS version possible is used between
  client and server. Currently stomp.py defaults to SSLv3 which is no
  longer secure.
- Add python-ssl as dependency for el5 (i.e. SL5) only as the ssl Python
  module was only introduced in Python 2.6 (SL6 and later) so we need the
  back-ported python-ssl package.