Fix Slack backend's use of deprecated method IM.OPEN

[frostyandy2k]

Error is thrown in certain cases where the send_messages function is called:
EXCEPTION: Slack API call to im.open returned an error: method_deprecated.

Similar issue description can be found here: errbotio/errbot#1431

There is also a fix that can be ported to this repo (API update): errbotio/errbot#1432

response = self.api_call('im.open', data={'user': id_})
->
response = self.api_call('conversations.open', data={'users': id_})