Bump suggested node version, add bin and license keys to package

[quaelin]

• Bump the suggested Node version in .nvmrc to the latest, but leave the "engines" Node version as >=4.4.4 for broader compatibility.
• Specify license as MIT. (Still need to finish code and legal review before we can make this public, though.)
• Add "bin": "./index" to package.json, so that npm will install redis-key-scanner as an executable.

@erikogan @mertonium

[quaelin]

Aha, and I was just reminded why I didn't have this running with Node 6.x. nodejs/node#6297 means that the console.log(usage) statement ends up getting truncated.

So for now, setting recommended version back to 4.4.5.

[erikogan]

I don’t think this should block release, but nodejs/node#6456 suggests a workaround to ensure that stdout/err get flushed before exit. We should probably look at setting them as blocking.

(Even though we recommend 4.4.5, people will run on whatever they have (or whatever is easiest to install, which would be latest))

[quaelin]

Ok @erikogan, took your suggested workaround. I've confirmed it works with both 4.4.5 and 6.2.0.

[mertonium]

[erikogan]

LGTM. @quaelin++ for no additional tech-debt