Support Node 8.10

[pselle]

The install script targets 6.10:

https://github.com/iopipe/v8-profiler-lambda/blob/master/package.json#L34

Given 8.10 support on Lambda, this should be more nuanced to support 6 & 8 engine profilers

cc @ewindisch am I on the right track here?

[pselle]

In investigating, there's not a pre-built for 8.10, and it doesn't appear that v8profiler is moving to make this available -- perhaps we should host our own pre-built?