Accept fraction with just a numerator

[philipnbbc]

Details

This PR supports reading fractions with an optional denominator (which defaults to 1). It does not change writing fracions to avoid breaking cases where the denominator is not optional in the JSON schema.

The TAMS for example has frame_rate specified with an optional denominator.

Pivotal Story

Story URL: Part of https://www.pivotaltracker.com/story/show/187542840

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