Explicit configuration for dependabot

[wilsonge]

• Create a dependabot.yml configuration file
• Remove Greenkeeper references (I've just uninstalled the extension as it's been discontinued for a year now)

[dgrammatiko]

@wilsonge instead of dependabot you should use renovate: https://www.whitesourcesoftware.com/free-developer-tools/renovate/ (also free but way better than dependabot)