Fix issue #257 with upgrade to RxJava 0.18.2

Fixes https://github.com/Netflix/Hystrix/issues/257 by upgrading to RxJava 0.18.2 with a fix found via Hystrix.
Merge pull request #259 from benjchristensen/rxjava-0.18.2

Fix #257 with RxJava 0.18.2