Make http request async

raven:capture/2 blocks until http request finishes. It can slow
down error_logger when raven is used with {error_logger, true}
because error_logger waits until raven_capture/2 finishes.

Currently raven:capture/2 does not check return value of http
request so it's safe to make http request async.
Add support for IPv6.

 Fix issue #10
Update README to document ipfamily env var.
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.
Use with lager via raven_lager_backend