Fixtures and helpers are not compiled via Babel

[vadimdemedes]

I've used default function parameters while testing code on Node.js v4 (which does not support them) and discovered that AVA does not compile helpers, fixtures and underscore-prefixed files (e.g. _setup.js).

I compile the library source via Babel and AVA takes care of test files, but using "next-gen" code in test helpers breaks the compatibility.

The solution to this would be to include tests, fixtures and _*.js files into precompilation, but still ignore those files (not treat them as tests).

[novemberborn]

See #720.