Allow prereleases of rspec 3.0.0

If you use guard-rspec with rspec 3.0.0.beta1, bundler will downgrade guard-rspec all the way down to 1.2.1 because that was the last version that didn't specify a dependency on rspec. 1.2.1 works just fine for most purposes, but I couldn't figure out how to use it with [spring](https://github.com/jonleighton/spring) without the `cmd` parameter. This commit allows us to use guard-rspec with prereleases of rspec 3.0.0 until such time as it is stable.