Update ava to the latest version 🚀 (#31)

Author: greenkeeper[bot]

package.json

@@ -22,7 +22,7 @@
     "sugarml": "^0.6.0"
   },
   "devDependencies": {
-    "ava": "^0.20.0",
+    "ava": "^0.21.0",
     "coveralls": "^2.13.1",
     "nyc": "^11.0.3",
     "reshape": "^0.4.0",