ci(gemfile): update for Vagrant testing [skip ci]

* Automated using https://github.com/myii/ssf-formula/pull/125

Gemfile

@@ -5,7 +5,12 @@ source 'https://rubygems.org'
 gem 'kitchen-docker', '>= 2.9'
 gem 'kitchen-inspec', '>= 1.1'
 gem 'kitchen-salt', '>= 0.6.0'
+gem 'rspec-retry'
 # Latest versions of `train` cause failure when running `kitchen verify`
 # Downgrading to `3.2.0` until this is fixed upstream
 # https://github.com/inspec/train/pull/544#issuecomment-566055052
 gem 'train', '3.2.0'
+
+group :vagrant do
+  gem 'kitchen-vagrant'
+end