Updated rebar to a new version

The rebar version when I checkout Emysql didn't seem to recognize pre-compile hooks. When I checked the version, it was a pretty old version (20101010_193118). I updated it to the most recent one from the canonical rebar repo's README page (https://github.com/rebar/rebar, version 20120604_145614). After this, everything builds smoothly, without needing to separately invoke crypto_compat.escript.