Move puppet libary target to /Library/Ruby/Site

We currently drop the puppet library files in /usr/lib/ruby/site_ruby/1.8 on
OSX, which is a symlink to /Library/Ruby/Site/1.8. As of OSX 10.9 Mavericks,
this will no longer work because it will be ruby 2.0.0. There are hard ways and
easy ways to solve this problem. A hard way would be to somehow introspect the
target host at install time, and then dynamically copy the payload into the
correct ruby version specific path. Another hard way would be to create
different OSX packages for different versions of OSX. Or, we could pick the
easy way, which is to just install into /Library/Ruby/Site (the equivalent of
/usr/lib/ruby/site_ruby). Since puppet will work on all rubies shipped with
supported versions of OSX, this will "just work".

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>