Update Rspec syntax

Update Rspec syntax

Did this with the help of the transpec gem. Contrary to the README, I
found I did need to add it to the Gemfile (although obviously, I removed
it immediately afterwards).

Additionally it crashed trying to open `.git/COMMIT_EDITMSG` after the
conversion had completed, because I was running it from inside a
submodule; upstream issue for that:

  https://github.com/yujinakayama/transpec/issues/94

Here are the relevant bits of the output:

  Summary:

  415 conversions
    from: obj.should
      to: expect(obj).to
  382 conversions
    from: == expected
      to: eq(expected)
  100 conversions
    from: lambda { }.should
      to: expect { }.to
  35 conversions
    from: obj.should_not
      to: expect(obj).not_to
  32 conversions
    from: lambda { }.should_not
      to: expect { }.not_to
  2 conversions
    from: it { should_not ... }
      to: it { is_expected.not_to ... }
  1 conversion
    from: it { should ... }
      to: it { is_expected.to ... }

  967 conversions, 0 incompletes, 0 warnings, 0 errors

Sure beats doing it by hand, or even with Vim macros...