Updating MultiJSON dependency to 1.9.0

Previously required version of MultiJson (1.3.4) does not have support for ParseError.

But Gibbon depends on it in at least one place:  https://github.com/amro/gibbon/blob/master/lib/gibbon/api_category.rb#L35
``rescue MultiJson::ParseError``

The ParseError module didn’t exist (or was called LoadError) until MultiJson v 1.9.0: https://github.com/intridea/multi_json/blob/master/CHANGELOG.md

You would need at least MultiJson 1.9.0 for current release to not have this bug.