Update ava to version 2.4.0 (#302)

* chore(package): update ava to version 2.4.0

Closes #280

* Fix ava tests

Co-authored-by: greenkeeper[bot] <23040076+greenkeeper[bot]@users.noreply.github.com>

Author: RyanZim

package.json

@@ -32,7 +32,7 @@
     "yargs": "^15.0.2"
   },
   "devDependencies": {
-    "ava": "^1.0.1",
+    "ava": "^2.4.0",
     "coveralls": "^3.0.0",
     "eslint": "^5.0.0",
     "eslint-config-problems": "2.0.0",
@@ -70,5 +70,10 @@
     "url": "https://github.com/postcss/postcss-cli/issues"
   },
   "homepage": "https://github.com/postcss/postcss-cli#readme",
-  "license": "MIT"
+  "license": "MIT",
+  "ava": {
+    "helpers": [
+      "test/helpers/*"
+    ]
+  }
 }

test/error.js

@@ -56,7 +56,7 @@ test('plugin throws on require', t => {
   return cli([
     'test/fixtures/a.css',
     '-u',
-    './test/fixtures/bad-plugin',
+    './test/fixtures/_bad-plugin',
     '-o',
     tmp()
   ]).then(({ error, code }) => {