Added lodash.
Added PUT for resource /retrospectives/:retroId/tickets/:ticketId
Added handler method putTicket
Updated wrapper method put to not delete empty fields.
 - And added new field 'updatedAt'

Summary:

 A get is called first to collect all existing values, then a put is called to
 to update the required fields, finally a get is called to return the updated document.
 We can probably optimize this later.