Pin Mocha development dependency.

Version 1.0.0 of Mocha appears to have backwards-incompatible changes;
pinning these for now to fix the build; we can diagnose the breakage in
more depth later.

Also fix the `require` statement in the test setup, which triggers a
deprecation warning.
Merge pull request #10 from alphagov/pin-mocha

Pin Mocha development dependency