Replaced rbx-2 with rbx-2.2.10.
Fix: mutually_exclusive and exactly_one_of validation error messages now label parameters as strings, consistently with requires and optional.
Add JSON output to Grape::Exceptions::ValidationErrors.
Merge pull request #687 from dblock/consistently-label-errors

Fix: mutually_exclusive and exactly_one_of validation error messages now label parameters as strings, consistently with requires and optional.