specify required ruby version

[timoschilling]

Kaminari 1.0 works only with Ruby >2, this helps Bundler to find Kaminari versions for Ruby 1.9

[amatsuda]

@timoschilling Well, I think you made a good point, but since we've already released 1.0.0 that doesn't include this directive, I'm afraid adding this to the next release won't actually help the 1.9 users to find the one that works for them.

I'm wondering if we'd better have this in the gemspec still...

[timoschilling]

Yes it don't solve any Problem, but it will help in some cases, for example a Gem which uses kaminari can say kaminari !=1.0.0 and it will work. ActiveAdmin has this problem.

[amatsuda]

All right, I'll merge this. TBH I'm still not 100% sure how would this help the ActiveAdmin's case, but it shouldn't be a bad thing to have in our gemspec anyway.
Thank you!