Release 0.6.0

Summary:
This feature release adds the `validate` rake task and the ability to
test strict variables and the future parser with rspec-puppet.

Features:
- Add `validate` rake task.
- Add `STRICT_VARIABLES=yes` to module_spec_helper
- Add `FUTURE_PARSER=yes` to module_spec_helper

Bugfixes:
- Avoid conflict with Object.clone
- Install forge fixtures without conflicting with already-installed
  modules