chore(package): update ava to version 0.21.0

Closes #17

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "@aureooms/js-operator": "^1.0.2",
     "@aureooms/js-random": "^2.0.0",
     "@aureooms/js-string": "^0.2.0",
-    "ava": "^0.19.1",
+    "ava": "^0.21.0",
     "babel-cli": "^6.24.1",
     "babel-polyfill": "^6.23.0",
     "babel-preset-env": "^1.4.0",