JVMCBC-47: Update RxJava to RC7.

Motivation
----------
1.0.0 GA shipped with RC3 and the plan is to make sure the code
stays as close as possible with bugfix releases until RxJava
hits 1.0.0 GA as well.

Modifications
-------------
Move to RC7.

Along the way, a assertion error was discovered where the abstract
loader did subscribe twice to the same scheduler.

Result
------
More bugfixes for users and us in RC7.

Change-Id: Ic26c882a9dd80948ac38572421154e7d1abba89f
Reviewed-on: http://review.couchbase.org/42321
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>