Fixed a bug with jiffy and GCC4.8

Basically, GCC4.8 with -Wall (out of our control) was detecting a declaration-never-used as a warning, which was treated as an error.

Newer version of jiffy resolves it, and added the `rm -rf deps` bit because if you have a deps and run `./rebar get-deps`, it doesn't fetch even if you have a different version in your `rebar.config` than the version in `deps`.