Update dependencies to enable Greenkeeper 🌴 (#82)

* chore(package): update dependencies

* docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # fast-json-stringify  [![Build Status](https://travis-ci.org/fastify/fast-json-stringify.svg?branch=master)](https://travis-ci.org/fastify/fast-json-stringify)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/fastify/fast-json-stringify.svg)](https://greenkeeper.io/)
+
 __fast-json-stringify__ is significantly faster than `JSON.stringify()` for small payloads. Its performance advantage shrinks as your payload grows. It pairs well with [__flatstr__](https://www.npmjs.com/package/flatstr), which triggers a V8 optimization that improves performance when eventually converting the string to a `Buffer`.
 
 Benchmarks:

package.json

@@ -29,7 +29,7 @@
     "is-my-json-valid": "^2.17.2",
     "long": "^4.0.0",
     "pre-commit": "^1.2.2",
-    "proxyquire": "^1.8.0",
+    "proxyquire": "^2.0.0",
     "standard": "^11.0.0",
     "tap": "^11.1.1",
     "uglify-es": "^3.3.10"