Update dependencies to enable Greenkeeper 🌴 (#1)

* chore(package): update dependencies

* docs(readme): add Greenkeeper badge

package.json

@@ -34,10 +34,10 @@
     "pkg-dir": "^2.0.0"
   },
   "devDependencies": {
-    "ava": "^0.19.1",
+    "ava": "^0.21.0",
     "coveralls": "^2.13.1",
     "eslint-config-xo-lukechilds": "^1.0.0",
-    "nyc": "^10.3.2",
+    "nyc": "^11.0.3",
     "xo": "^0.19.0"
   }
 }