fix: disable the babel/nyc cache

[bcoe]

the interactions between the ava, babel, and nyc cache were causing a lot of confusion for folks, see:

#296
#294

In this pull request:

• we leave nyc's cache as opt in (I was considering making it run by default in 7.x).
• we disable babel's cache when running tests with nyc (this cache caused a lot of issues with babel-plugin-istanbul).

[bcoe]

@jamestalmage, @JaKXz, @gotwarlost, @kentcdodds mind giving the release candidate 7.0.0-alpha.5 a spin, adds:

• @jamestalmage's fixes for pid collisions.
• fixes for babel cache (which was making babel-plugin-istanbul/__coverage__ hard to use).
• instrument command for instrumenting files outside the context of tests.

If everyone signs off, I'd like to push this to next soon -- back-peddled a bit on the first release because of various issues opened around caching.

[JaKXz]

@bcoe works for me 👍 [on OS X]

[kentcdodds]

nyc@next is working great for me too!

[gotwarlost]

sorry for not responding. I was on a flight from India and dealing with jet lag later. Sounds like it's rocking now! I'll give it a whirl.