Add Ruby 1.8 hash syntax

For my sins, I'm working in a codebase based on Ruby 1.8 / Rails 2.3, I need to add an AWeber integration to it, and your gem saved me the hassle of writing an OmniAuth strategy myself. It works perfectly, aside from the 1.9 hash syntax in the OmniAuth strategy configuration. A trivial change, but it would be nice to be able to use the gem instead of adding the strategy into my app's lib directory.

My PR simply modifies the configuration hash syntax to go to the old 1.8 hashrocket style.

Thanks for the gem!