pkg-rebar: add bin to list of install directories

Some Erlang packages provide a bin directory for programs that are to be
invoked from the command line. An example of such a package is the Lisp
Flavored Erlang compiler. The Erlang OTP library includes several more
examples (it doesn't use rebar, though.) This change makes sure that the
bin directory gets installed too.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>