Add ability to use default homebrew install location by setting a custom fact.

If the "use_default_homebrew" fact exists, the module will do the following:
 * Set homebrew directory to /usr/local
 * Remove custom boxen install commands
 * Set the provider to use standard install commands
 * Add a new custom "latest" command to homebrew to get the latest version of a package

This is desirable for many people as discussed at https://github.com/boxen/puppet-boxen/issues/20.