Update rebar from binary under riak

After cloning riak develop branch, `make` fails with the following error message:

```
ERROR: Failed to get ebin/ directory for goldrush escript_incl_apps.ERROR: escriptize failed while processing /home/shino/work/basho/git/riak-2.0/deps/cuttlefish: rebar_abort
ERROR: Command [compile] failed!
make: *** [compile] Error 1
```

$ ./rebar --version
rebar 2.1.0-pre R14B04 20131017_164918 git 2.1.0-pre-168-gaa46d85

(I don't yet dig into details of the error, but I think faster is better)