ci(gemfile): restrict train gem version until upstream fix [skip ci]

* Automated using myii/ssf-formula#110

Gemfile

@@ -5,3 +5,7 @@ source 'https://rubygems.org'
 gem 'kitchen-docker', '>= 2.9'
 gem 'kitchen-inspec', '>= 1.1'
 gem 'kitchen-salt', '>= 0.6.0'
+# 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'