Upgrade ava to 1.x beta (#905)

Since in the words of the maintainers:

> This release uses babel@v7.0.0-beta.44. We're waiting for Babel 7 to
> go out of beta before releasing the 1.0 version of AVA itself. This
> doesn't mean that these releases are any less stable than our previous
> 0.x versions though! If your project itself doesn't use Babel, or if
> your tests aren't coupled to your use of Babel 6 you really should
> upgrade.

See:
https://github.com/avajs/ava/releases/tag/v1.0.0-beta.4

package.json

@@ -35,7 +35,7 @@
   },
   "devDependencies": {
     "auto-changelog": "^1.4.6",
-    "ava": "^0.25.0",
+    "ava": "^1.0.0-beta.4",
     "eslint": "^4.19.1",
     "eslint-config-prettier": "^2.9.0",
     "eslint-plugin-prettier": "^2.6.0",