Using older version of json gem

The library is using an older version of the json gem.
The newer version has some useful features (e.g. parsing a single value (not an object) correctly)
json 2.2.0
`````
JSON.parse("\"hello\"")
 => "hello" 
`````
json 1.8.6
`````
JSON.parse("\"hello\"")
Traceback (most recent call last):
        1: from (irb):1
JSON::ParserError (784: unexpected token at '"hello"')
`````
