Update Berkshelf and sensu-plugins-chef versions

[cwjohnston]

Description

• Update Berkshelf gem to ~> 5.6
• Update sensu-plugins-chef gem to 2.0.0

Motivation and Context

Current version of Berkshelf generates a lot of warnings (see https://github.com/berkshelf/berkshelf/issues/1669)

Current version of sensu-plugins-chef (installed in integration tests via sensu_test::gem_lwrp recipe) fails to install on Sensu 0.29 (Ruby 2.4) due to old version of ffi-yajl.

How Has This Been Tested?

Integration and unit tests pass.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.