Update rebar to rebar 2.1.0-pre R14B04 20130226_211548

This commit simply updates rebar with `rebar 2.1.0-pre R14B04 20130226_211548 git 2.1.0-pre-50-ge36b9c1` which is being used in riak, riak-cs, stanchion.  Builds were breaking because the previous rebar was passing incorrect 32-bit flags to gcc in erlang_js and leveldb.

## Checks

[X] - Fix broken eunit on builders?
[ ] - Dialyzer still works with new rebar?