Make test run identifying fields required

[davidjgoss]

The new fields introduced in #102 are optional. This was a tactical move at the time because making them required immediately would prohibit compatibility with any existing messages. Once this change has had plenty of time to be adopted by major Cucumber implementations and reporting tools, we should mark the fields as required.

The fields are:

• TestRunStarted.id
• TestCase.testRunStartedId
• TestRunFinished.testRunStartedId