[FIX] Use psycogreen instead of gevent-psycopg2

green-psycopg2 has been deprecated by the developer, he asks to use an
alternative, in this case, the alternative is psycogreen
https://bitbucket.org/dvarrazzo/psycogreen

bzr revid: stw@openerp.com-20130909131123-u0gqhvxqwv13yqke