Use `readable-stream` package for better node 0.8 compatibility.

This patch also ensures that images are fully written and the stream
closed before resolving the promise with their filename.

Change-Id: Id6498397e359a9bed5eeee7b30648931508db201
Add request timeouts, and retry failing requests.

Change-Id: Ic0bbba21b0dc139562deae6b69b650f501bf78d0
Make travis more patient.

The bundler can be slow; emit verbose output to ensure that builds
don't time out prematurely.

Change-Id: I966f6c21ec28591374a86b6d2f89ada47a498a8d