Upgrade node runtime to 10.x

[MatissJanis]

Node 8.10 is EOL. Amazon recently sent out an email encouraging customers to upgrade.

[MatissJanis]

Hey @jlengstorf

Would you mind taking a look at this? Node 8 is coming to EOL very, very soon in AWS lambdas.

[jlengstorf]

hey, @MatissJanis — I no longer work at Gatsby, so I don't control this repo anymore

maybe @gatsbyjs/core can take a look?

[pvdz]

@jlengstorf I guess github suggestions doesn't track people entering/leaving the org groups so it's still suggesting you as a reviewer, which might be what prompted the request for your review. Seems like an obvious case github could improve on.

@MatissJanis we're going to add a deprecation warning first, give people a little time to migrate their own env to node 10+. There are still >5% users on node 8! So please bear with us, we want to move forward as soon as possible as well but can't just cut over immediately :)