Ensure value is string before validating format

[melvey]

Format validation can only be applied to string objects. Checking the value is a string before validating the format means that format validation can be used when multiple types are accepted
e.g.

{
  type: ['string', 'null'],
  format: 'date-time'
}

[melvey]

Looks like the travis build fails to install dependencies as npm uses arrow functions which are not supported in the target node versions.
It might be worth updating the target build versions as a significant number of updates have been released since