Make metadata-json-lint into an implicit dependency

Renovate pushes gems to newer major versions, but that has caused a
dependency issue with voxpupuli-test. this latter gem is the one we
directly use for tests and it indirectly pulls in metadata-json-lint
with the version that it wants to use.

Gemfile

@@ -7,7 +7,6 @@ gem 'rake'
 
 group :tests do
  gem 'facter', '>= 2.4.0'
- gem 'metadata-json-lint', '~> 4.0'
  # Use info from metadata.json for tests
  gem 'puppetlabs_spec_helper', '>= 0.10.0'
  gem 'puppet-lint', '>= 2.3.0'