Second shot at having greenkeeper ignore eslint (#610)

First shot at getting greenkeeper to ignoring eslint updates landed with commit 85fa743.

Sadly that commit had a minor `greenkeper` misspelling in package.json, which obviously didn't have much effect for greenkeeper.
This commit fixes that blooper in hopes that greenkeeper will stop opening PRs about new eslint versions, as we don't want to
update at the moment cause eslint 3.x requires Node.js v4.x.

package.json

@@ -52,7 +52,7 @@
       "wrappers"
     ]
   },
-  "greenkeper": {
+  "greenkeeper": {
     "ignore": ["eslint"]
   },
   "license": "MIT"