Only set custom_octopus_connection in Rails 3.2

Here's an alternative solution to #170. We maintain the current behavior for 3.2, and ensure that the deprecated `octopus_establish_connection` method correctly sets `custom_octopus_connection` for Rails < 3.2.

I think this is more elegant long-term; I think it's best to have as few octopus-specific methods as possible.