Remove Node 4.3/6 support

[pselle]

https://docs.aws.amazon.com/lambda/latest/dg/runtime-support-policy.html

While invocations of Lambda functions configured to use Node.js v4.3 will continue to work normally, the ability to create new Lambda functions configured to use the Node.js v4.3 runtime will be disabled on July 31, 2018. Code updates to existing functions using Node.js v4.3 will be disabled on October 31, 2018.

Given AWS' stance on Node 4.3, we can remove support for this runtime from future IOpipe core

EDIT: add Node 6 @ewindisch