Update enterprise SSO URL regexp

[maciex]

New enterprise SSO URL is: 'https://xxx.enterprise.slack.com/sso/saml/start?redir=%2Fr-t1239467270%3Fredir%3D%2Fmessages'

This fixes #592

[raelgc]

Thanks @maciex. Just wondering if people there is a version without the enterprise word for simple plans. In this case, I prefer we make it a OR regexp to keep current and new.

[maciex]

So if we would like to have new and old ULR working the SSO url regexp should look like this: SSO_URLRE = re.compile(r'^http[s]://[a-zA-Z0-9-]+.(enterprise.)?slack.com/sso/saml/start(\?redir=.*)?$')

the services url regexp would be the same as in the pull request: SERVICES_URLRE = re.compile(r'^http[s]://[a-zA-Z0-9-]+.(enterprise.)?slack.com/services/.*')