Downgrade `readable-stream` dependency.

The docs for the `readable-stream` package say to use version "~1.0.0"
for Streams2 compatibility.  The 1.1.x is reserved for "streams3",
whatever that turns out to be.  At the moment, Streams2 is good enough
for us.

Change-Id: I179c8e885bc25f29bb169d4acdc8e06a4639378d