Omit .dice files in Bundler path or in dot-prefixed directories

This adds an implicit feature to ignore .dice files that have been installed with vendored gems in a subdirectory.

Cucumber tests are included. Testing was by far the most difficult part of this change. The tests run Bundler to install dice_bag's dependencies in a subdirectory so they are slower than other tests, but still only a few seconds.

I've also added a test to document the existing behaviour with dot-prefixed directories, as described in issue #69.