fix(gemfile): restrict inspec version to ~> 4.16.0

* Automated using myii/ssf-formula#45
* Current tests are resulting in `undefined method` errors with `4.17.7`:
  - https://travis-ci.com/saltstack-formulas/vault-formula/jobs/239671364#L2219-L2220
  - https://travis-ci.com/saltstack-formulas/vault-formula/jobs/239671365#L1925-L1926
  - https://travis-ci.com/saltstack-formulas/vault-formula/jobs/239671366#L1515-L1520

Author: myii

Gemfile

@@ -3,4 +3,5 @@ source "https://rubygems.org"
 gem 'kitchen-docker', '>= 2.9'
 gem 'kitchen-salt', '>= 0.6.0'
 gem 'kitchen-inspec', '>= 1.1'
+gem 'inspec',      '~> 4.16.0'