chore(greenkeeper): update the label used by Greenkeeper

...to "dependecies". As per (no official documentation as far as I know):
greenkeeperio/greenkeeper#153 (comment)

This brings it in line with the PRs generated by dependabot for security
fixes (e.g. #79). It is also just clearer in my opinion, especially for
people that don't know Greenkeeper.

greenkeeper.json

@@ -6,5 +6,6 @@
         "playground/package.json"
       ]
     }
-  }
+  },
+  "label": "dependencies"
 }