Declare Node v6 as minimum support

[lukeed]

The engines key will only show a warning to NPM installers. It's not perfect, but at least it's something.

Although we were compiling to Node 4 and our code worked in a Node 4 environment (I just tried, dat npm install doe...💤 ), we have several dependencies that require 5+ and 6+ and our dependency resolution was fighting against NPM's dependency tree (typical).

So with this, we're now compiling to Node 6 & actively declaring a 6+ minimum.

I removed Node 7 from Travis because we're already testing v8 and v7 literally brings nothing unique to the table. This will make tests completely a bit sooner.

References #165

[lukeed]

Oh, and normally this would be a breaking change for a library. But it isn't for us because we've already been informally requiring Node 6. This is just formally writing it down. 📝