Update dependencies to enable Greenkeeper 🌴 (#153)

* chore(package): update dependencies

* docs(readme): add Greenkeeper badge

* Revert "docs(readme): add Greenkeeper badge"

This reverts commit aefcc5a.

* revert abstract-leveldown

Author: greenkeeper[bot]

package.json

@@ -37,12 +37,12 @@
   },
   "devDependencies": {
     "airtap": "0.1.0",
-    "buffer": "~5.1.0",
+    "buffer": "~5.2.1",
     "coveralls": "^3.0.2",
-    "hallmark": "0.0.2",
+    "hallmark": "0.1.0",
     "level-community": "~3.0.0",
-    "nyc": "^12.0.2",
-    "standard": "^11.0.1",
+    "nyc": "^13.1.0",
+    "standard": "^12.0.1",
     "tape": "^4.0.0"
   },
   "hallmark": {