Use promise support in mocha.

Change-Id: I588f591a9d20cb61fd90b4e5c0d56944973747b7
Use `Promise.guard` from `prfun`.

Change-Id: I1fd92004a81a1cc27c19cb5f0a5e41d32c5503be
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