Update json requirement from ~> 2.1.0 to ~> 2.2.0

[dependabot-preview]

Updates the requirements on json to permit the latest version.

Changelog

Sourced from json's changelog.

2019-02-21 (2.2.0)

• Adds support for 2.6 BigDecimal and ruby standard library Set datetype.

2017-04-18 (2.1.0)

• Allow passing of decimal_class option to specify a class as which to parse
  JSON float numbers.

2017-03-23 (2.0.4)

• Raise exception for incomplete unicode surrogates/character escape
  sequences. This problem was reported by Daniel Gollahon (dgollahon).
• Fix arbitrary heap exposure problem. This problem was reported by Ahmad
  Sherif (ahmadsherif).

2017-01-12 (2.0.3)

• Set required_ruby_version to 1.9
• Some small fixes

2016-07-26 (2.0.2)

• Specify required_ruby_version for json_pure.
• Fix issue #295 failure when parsing frozen strings.

2016-07-01 (2.0.1)

• Fix problem when requiring json_pure and Parser constant was defined top
  level.
• Add RB_GC_GUARD to avoid possible GC problem via Pete Johns.
• Store current_nesting on stack by Aaron Patterson.

2015-09-11 (2.0.0)

• Now complies to newest JSON RFC 7159.
• Implements compatibiliy to ruby 2.4 integer unification.
• Drops support for old rubies whose life has ended, that is rubies < 2.0.
  Also see https://www.ruby-lang.org/en/news/2014/07/01/eol-for-1-8-7-and-1-9-2/
• There were still some mentions of dual GPL licensing in the source, but JSON
  has just the Ruby license that itself includes an explicit dual-licensing
  clause that allows covered software to be distributed under the terms of
  the Simplified BSD License instead for all ruby versions >= 1.9.3. This is
  however a GPL compatible license according to the Free Software Foundation.
  I changed these mentions to be consistent with the Ruby license setting in
  the gemspec files which were already correct now.

2015-06-01 (1.8.3)

• Fix potential memory leak, thx to nobu.

2015-01-08 (1.8.2)

• Some performance improvements by Vipul A M vipulnsward@gmail.com.
• Fix by Jason R. Clark jclark@newrelic.com to avoid mutation of
  JSON.dump_default_options.
• More tests by Michael Mac-Vicar mmacvicar@gmail.com and fixing
  space_before accessor in generator.
• Performance on Jruby improved by Ben Browning bbrownin@redhat.com.
• Some fixes to be compatible with the new Ruby 2.2 by Zachary Scott e@zzak.io

... (truncated)

Commits

• 6550c42 Merge branch 'master' of github.com:flori/json
• f53a0e3 It's more trouble than it's worth.
• 3631dad Use which to resolve to actual path
• 81fbce0 [fix] 2.5 compat on JRuby 9.2 Fixnum/Bignum -> Integer
• aff3d4b Always use underscore for match string
• 0e97fd4 Upgrade version to 1.6
• a0f3d12 Bump version to 2.2.0
• 3eb7e9e Add some missing ruby 2.6 changes
• bb80864 Fix a typo.
• ef2092f Fix for bigdecimal updates
• Additional commits viewable in compare view

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