More adjust rspec3

I happen to see this PR (https://github.com/sanemat/tachikoma/pull/54), and I know a540e77 is too edge way :sweat: 

When rspec3.0.0.beta2 is released, a540e77 should be reverted and require rspec3.0.0.beta2 in gemspec will work fine.
(it seems https://github.com/rspec/rspec-core/blob/master/Changelog.md#300beta2-development 3.0.0.beta2 will release soon?)