Enable caching of busser gems only if we are in a test-kitchen run

[tknerr]

This is a hack-like workaround to limit caching of the generic busser gems cache to occasions when we are running as part of a test-kitchen run.

We use Dir.pwd to guess whether we are in a kitchenci generated directory and thus can assume we are running in a kitchenci run with the kitchen-vagrant plugin.

Why doing this? Because

1. we only need it in this case and
2. it alleviates problems with windows guests (see Windows Guest Support for Generic Bucket (and others too?) fgrehm/vagrant-cachier#137)

[tknerr]

Also added compatibility with test-kitchen 1.4, see test-kitchen/test-kitchen#677 (comment)

[tknerr]

Also changed the cached directory to /tmp/busser|verifier/gems/cache - otherwise we would never get an updated busser version itself according to this.