Upgrade to RSpec 3

It's good to keep up to date.  RSpec 3 has also deprecated all the monkey-patching of methods onto `Object` in favour of the new `expect(subject).to eq(something)` style syntax.
I had a look at the history of the gem, and it seemed reasonable to lock to the minor version only. They seem to be doing only bugfixes on the patch versions.  Having said that, there's no real reason not to lock it down more.  I'll revert this change.