Update master with mainline rebar (v2)

This PR uses the latest software provided by the upstream Rebar (v2) repo.

The most significant change allows this version of Rebar to work with
compilers which do not support the -m64 flag, as reported in #892
amongst others.

The latest code base also contains @davisp's Properly skip apps with a
.app.src.script file patch which CouchDB has been carrying for a while
now.