Delete rebar

This is an older version of rebar:
rebar 2.0.0 R14B02 20120717_213113 git 2.0.0-99-ga8d11dc

It's causing problems in our build (specifically, raven fails to get it's jiffy dependency).
Update jiffy version 0.11.3 -> 0.13.1

Raven is failing to fetch jiffy.  Hoping that updating the version will fix.