chore(package): update ava to version 0.21.0

Closes #19

Author: greenkeeper[bot]

package.json

@@ -28,7 +28,7 @@
     "@aureooms/js-compare": "^1.4.5",
     "@aureooms/js-itertools": "^3.1.1",
     "@aureooms/js-random": "^2.0.0",
-    "ava": "^0.19.0",
+    "ava": "^0.21.0",
     "babel-cli": "^6.23.0",
     "babel-polyfill": "^6.23.0",
     "babel-preset-latest": "^6.22.0",