Replace map-stream with regular TransformStream

Reverts #11. According to #15 the returned map-stream doesn't implement the expected
Readable interface, thus we're switching back to node's TransformStream.

Fixes #15
v1.0.5
Replace heartbeat with ping-pong

Node is now waiting for Phantomjs to respond with a 'pong' before sending the next 'ping'.
This could fix performance issues when Phantomjs' stdin was cluttered with heartbeat messages.

Fixes #12
v1.0.6