change default_directory to default_path

- Closes #397
Change feature filename
relish .nav
ci: build against jruby is hanging on travis, so removing for now
--default_directory is not supported on Ruby 1.8.6
Support default_path option across all supported rubies

The issue was the non-deterministic order of hash keys.
less flattening
ignore .rbc files
dev: Rakefile tweaks
dev/ci: add binstubs to 'b' directory and tell travis to run b/rake

Tried to move this gem's bin dir to exe and store binstubs in bin, but
that resulted in a stack overflow error.
dev: add rdoc to Gemfile
dev: remove dep on rdoc
one more binstub
dev: constrain file list in gemspec
Ensure let methods return a cached nil value instead of repeatedly evaling block
Minor adjustments to let_spec.rb

- #405
Changelog

- #405
fix failing formatter specs
fix failing formatter specs in 1.8.7
ci: pull 1.9.1 from travis build - upstream deps are causing errors