Update tests with correct results for unfound keys
Return nil instead of false for unfound attributes

"nil" is the correct return for attributes that are not found. Otherwise
hiera will neither fail as it should nor return a supplied default.
Add default value test for previous bug

This test would have turned up the bug fixed in the previous commit more
obviously than the tests I just fixed.
Replace Modulefile with metadata.json
Add hack rake tasks to work around changing hiera_config
Add fixtures
Add gemfile for CI testing
Add config for travis