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.
Considering Timecop hasn’t hit `1.0.0` yet, either it’s not following SemVer, in which case we should lock it to a specific version, or it is following SemVer and is allowed to break whatever it likes until `1.0.0`, in which case we should lock it to a specific version.