Changed version number in README.md to use the same as version.rb

Hey,

I realize that you are using README driven development for Resque 2.0, but I think it'd be useful to have the right version number in there.

I am currently migrating from 1-x-stable to 2.0 because I need one of the latest patches. 

What do you think @steveklabnik?

Thanks,
Ernesto