Relax gemspec constraints on rake, rspec and minitest-chef-handler

Hey @sethvargo,

I'm submitting this PR to relax some of the gemspec constraints in chefspec, as a point of discussion. I don't think the minitest-chef-handler one should be too controversial since 1.0.0 was just released, but I'm unsure about rake and rspec. Is there a reason you pinned them as so?

I removed all the constraints here and ran the tests under Bundler, thereby using the following:

    rake (10.0.4)
    rspec (2.13.0)
      rspec-core (~> 2.13.0)
      rspec-expectations (~> 2.13.0)
      rspec-mocks (~> 2.13.0)
    rspec-core (2.13.1)
    rspec-expectations (2.13.0)
      diff-lcs (>= 1.1.3, < 2.0)

and everything passed.