Updated the minimum version of sensu-plugin

[madelaney]

Pull Request Checklist

This pull-request is intended to address bug #61

Prior to the change, when trying to run the Graphite mutator, or any function, the following stack trace would get thrown

/opt/sensu/embedded/bin/handler-graphite-notify.rb --help
/opt/sensu/embedded/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:2278:in `check_version_conflict': can't activate json-1.8.6, already activated json-2.0.2 (Gem::LoadError)
    from /opt/sensu/embedded/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:1404:in `activate'
    from /opt/sensu/embedded/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:89:in `block in require'
    from /opt/sensu/embedded/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:88:in `each'
    from /opt/sensu/embedded/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:88:in `require'
    from /opt/sensu/embedded/lib/ruby/gems/2.4.0/gems/sensu-plugins-graphite-2.3.0/bin/mutator-graphite.rb:35:in `<top (required)>'
    from /opt/sensu/embedded/bin/mutator-graphite.rb:22:in `load'
    from /opt/sensu/embedded/bin/mutator-graphite.rb:22:in `<main>'
Exit 1

After the change, we get the expected data

/opt/sensu/embedded/bin//handler-graphite-notify.rb --help
Usage: ./handler-graphite-notify.rb (options)
    -j, --json_config JsonConfig     Config Name

General

• [ ] Update Changelog following the conventions laid out on Keep A Changelog

• [ ] Update README with any necessary configuration snippets

• [ ] Binstubs are created if needed

• [ ] RuboCop passes

• [ ] Existing tests pass

New Plugins

• [ ] Tests

• [ ] Add the plugin to the README

• [ ] Does it have a complete header as outlined here

Purpose

Known Compatablity Issues

[madelaney]

@majormoses My apologies for the incorrect labeling.

[majormoses]

No worries, every project/community has it's own shenanigans.

[majormoses]

released: https://rubygems.org/gems/sensu-plugins-graphite/versions/3.0.0