1.8.7 Compatibility

[phillbaker]

I know 1.8.7 is on it's way out, but define_singleton_method in looks like the only thing making this gem incompatible with 1.8.7.

Any chance of accepting a pull request that replaces define_singleton_method? (intercom/intercom-rails#31 would be a similar example of a replacement.)

[clbustos]

Personally, I don't have any problem on create the method on-the-fly for 1.8.7.