Simplify code coverage recipe
Simplify code coverage recipe

nyc is now officially part of Istanbul. They also have a tutorial on how
to integrate it with Babel. We should simplify the recipe so it's only
about using nyc with AVA. I've left the .gitignore bit, but removed
Coveralls since it's not directly related to AVA.

avajs/ava,sindresorhus/ava,avajs/ava