Update to rspec3

WHAT?
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The gemspec doesn't specify the rspec version for oat. When I checked out out oat and ran `bundle install`, bundler installed rspec 3. Unfortunately the tests raise errors, and the tests don't run.

This PR upgrades oat to use rspec3

HOW?
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To make this work, I've added the new rspec-its gem, so that examples that use `its` still work. I've also converted the `should` expectation syntax to the `expect` expectation syntax in a couple of places (to remove warnings)