Use drafter graph version instead of modified date

[Robsteranium]

Per @callum-oakley's comment on the graph-modified-time's PR:

We're using the modified times to determine if a graph has changed, but technically there's a race condition here if a graph changes twice within a millisecond (which appears to be likely to actually happen). The solution to this would be to use versions instead, which drafter now uses internally but which are not currently exposed by the public modified time graph. It would be a fairly small change to expose them and then update this work to make use of them.