Calling `gramps()` with no arguments causes an error

[jlengstorf]

Per #81, it looks like we added a regression when we extracted prepare() into its own function. The gramps() call no longer works without arguments.

This is minor, but something I'd like to fix.

[ryanomackey]

Looks like that wasn't the actual issue. I was just running node 6.9.5. Would it make sense to include an engines field in the package.json? And maybe a prerequisite in the docs?

[jlengstorf]

Maybe we should just transpiled back to Node 6?