Drop support for Node 4 and Node 6

[ide]

Jest no longer supports Node 4 so we need to test on Node 6+, and Node 8 is LTS anyway so may as well make the jump. We can continue to let this package run on Node 4 and 6 if it happens to -- just that new versions that don't work on Node 4 or 6 won't be rolled back. Making this change and upgrading spawn-async in Expo CLI would raise the minimum Node version to run it so we should be deliberate about this change.