ava: regression: array.includes no longer working.

Reference: https://github.com/sindresorhus/serialize-error/pull/3#discussion_r45643804

The following no longer works:

test(t => {
  var keys = Object.keys(obj);
  var hasName = keys.includes('name');
  t.true(hasName);
  t.end();
});

I get the following error:

TypeError: keys.includes is not a function

We are using the runtime plugin, to avoid any polyfills, so it makes sense that Array.prototype.includes does not actually exist. I would have thought the runtime plugin would have picked this up and provided it via some helper function.

We are still on Babel 5, so I am punting until we land Babel 6 support before digging in.

// @sebmck @thejameskyle