Fix npm warnings during "npm install"
$ npm install
npm WARN package.json carbon-footprint@4.0.1 No repository field.
npm WARN package.json carbon-footprint@4.0.1 No license field.
npm WARN engine web-ext@1.10.1: wanted: {"node":">=6.0.0","npm":">=3.0.0"} (current: {"node":"4.2.1","npm":"2.14.7"})
npm WARN engine dispensary@0.10.10: wanted: {"node":">= 6.0.0"} (current: {"node":"4.2.1","npm":"2.14.7"})
npm WARN engine hawk@6.0.2: wanted: {"node":">=4.5.0"} (current: {"node":"4.2.1","npm":"2.14.7"})