update gemspec for correct decent exposure version

I've accidentally mandaged to bundle a project using decent_decoration against decent_exposure 1.x version - it caused whole thing to fail, since DE 1.x series does not support any option hash for expose method 

1.x https://github.com/voxdolo/decent_exposure/blob/v1.0.2/lib/decent_exposure.rb#L19
vs 
2.x https://github.com/voxdolo/decent_exposure/blob/v2.0.1/lib/decent_exposure/expose.rb#L39

Let's set the minimum version of DE to 2.0 so other can avoid this :)