Upgrade to node 4

[technicalpickles]

Noticed this from Heroku while deploying:

remote: -----> Installing binaries
remote:        engines.node (package.json):  0.10.x
remote:        engines.npm (package.json):   unspecified (use default)
remote:
remote:        Resolving node version 0.10.x via semver.io...
remote:        Downloading and installing node 0.10.48...
remote:        Using default npm version: 2.15.1

Node 4, because that is what we are targetting as part of hubotio/evolution#4