Bump ava from 3.14.0 to 3.15.0

Bumps [ava](https://github.com/avajs/ava) from 3.14.0 to 3.15.0.
- [Release notes](https://github.com/avajs/ava/releases)
- [Commits](avajs/ava@v3.14.0...v3.15.0)

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -26,7 +26,7 @@
   "homepage": "https://github.com/alvarocastro/placeholdify#readme",
   "dependencies": {},
   "devDependencies": {
-    "ava": "3.14.0",
+    "ava": "3.15.0",
     "coveralls": "3.1.0",
     "nyc": "15.1.0",
     "semantic-release": "^17.3.2",