Update engines directive in package.json

[Steven Barlow]

Any reason why the package.json engines directive cannot be updated for Node 4.x to prevent the npm warning: follow@0.12.1: wanted: {"node":"0.12.x || 0.10.x || 0.8.x"} (current: {"node":"4.2.0","npm":"2.14.7"})?

Obviously this assumes that testing on Node 4.x proves compatibility.