Only call getRemainingTimeInMillis if exists
Only call getRemainingTimeInMillis if exists

Some mocking libraries do not implement getRemainingTimeInMillis,
guard against this and don't break just because a mocking library
is incomplete!

Fixes bug #34

Signed-off-by: Eric Windisch <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@iopipe.com>

pselle/iopipe,iopipe/iopipe-agent-node,iopipe/iopipe