Remove dependency on causes.git async_dispatcher

The fact that we happen to use `#async_send` for dispatch in causes.git
is an irrelevant implementation detail which caused the specs to fail.
Remove the dependency.

Change-Id: Ic9d00db860a4e37fbd63e1b0e8267d694ee453cb
Reviewed-on: https://gerrit.causes.com/26276
Reviewed-by: Lann Martin <lann@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>