test_isolate: update raindrops to 0.11.0

raindrops 0.11.0 was released several days ago and contains
minor improvements + fixes.
SIGNALS: document SIGHUP/preload_app incompatibility

This was documented in the unicorn docs, but not Rainbows!
(The major difference between unicorn and Rainbows! signal
handling is the deferred handling of SIGUSR1 in unicorn vs
the immediate handling in Rainbows!)