Revert back to using rd_kafka_conf_set_dr_cb() which is 0.8.3 compatible

Fixes #36
Bundle a known good version of librdkafka (0.8.4) with Hermann

This reduces the need for the librdkafka system dependency and results in
statically linking librdkafka.a against the C extension.

There's a few hacks in extconf.rb specifically to deal with librdkafka's build
system, as well as apply checksumming to the downloaded file

Fixes #8
Roll back to using 0.8.4 APIs since we're bundling now
Disable rubinius builds in travis
Merge pull request #38 from rtyler/issues/8-bundle-librdkafka

Bundle librdkafka 0.8.4 with the gem