Replaced json dependency with json_pure.

[trevorrowe]

Removed hard dependency on json >= 1.8.1. Replaced with json_pure >= 1.8.1. The runtime will still attempt to load the faster gem, if availble and will fall back on json_pure for compatability. Ruby 2.0+ ships with 1.8.1 by default, so only Ruby 1.9.3 will default to the slower version.

See #18

[xeron]

Unfortunately with json_pure dependency we're getting issues like this now (using aws-sdk and json inside Chef):

TypeError: wrong argument type JSON::Pure::Generator::State (expected Data)