replace process.nextTick with setImmediate to fix 'RangeError: Maximum call stack size exceeded'

[node-unzip](https://github.com/nearinfinity/node-unzip) depends on node-pullstream, but unzipping certain archives generates a bunch of:

>(node) warning: Recursive process.nextTick detected. This will break in the next version of node. Please use setImmediate for recursive deferral.

Followed by:

>RangeError: Maximum call stack size exceeded

node-unzip had a similar problem in its own code and fixed it by replacing *process.nextTick* calls with *setImmediate* (nearinfinity/node-unzip#28). This PR does the same for node-pullstream and fixes the problem unzipping files in my testing.