Add support for InSpec 4

[aaron-lane]

Detailed Description

inspec-gcp is currently pinned to InSpec 3, which prevents it from working with InSpec 4. The pinning should be updated to include InSpec 4.

Context

Kitchen-Terraform 5 updated to InSpec 4, which means that users can not presently use inspec-gcp. inspec-gcp: Can't find train plugin gcp.

Possible Implementation

Change the dependency pinning to '>= 3.0.25', '< 5'.

[skpaterson]

Thanks for raising this @aaron-lane. The handling of attributes/inputs has changed in InSpec v4:
inspec/inspec#4260
I'll update the pinning as soon as this is resolved InSpec side (or by reworking the controls here).

[skpaterson]

Turned out ^ was a non-issue so we can proceed. As per the instructions here:
https://github.com/inspec/inspec#install-it-via-rubygemsorg
the pinning will be updated to:

gem 'inspec-bin'