Change visibility of `providers` and `booted` to protected
merged branch gigablah/property-visibility (PR #665)

This PR was merged into the master branch.

Discussion
----------

Change visibility of `providers` and `booted` to protected

This changes `$providers` and `$booted` in `Application` to protected, which makes it easier to extend.

Commits
-------

e6902d1 Change visibility of `providers` and `booted` to protected