Upgrade JSON Pure gem to prevent deprecation warnings
Upgrade JSON Pure gem to prevent deprecation warnings

A lot of these were printed when I ran the specs:

    [snip]/json_pure-2.1.0/lib/json/common.rb:156: warning: Using the
    last argument as keyword parameters is deprecated

I think this is unlikely to break anything. This gem is only used by
the Chargebee gem.

https://github.com/flori/json/blob/master/CHANGES.md

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