Add ability to use default homebrew install 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
Address puppet lint complaints
Merge remote-tracking branch 'ssgelm/master'