Update dependencies to enable Greenkeeper 🌴 (TypeStrong#490)

Author: greenkeeper[bot]

Diff for: README.md

@@ -5,6 +5,7 @@
 [![Build Status](https://travis-ci.org/TypeStrong/typedoc.svg?branch=master)](https://travis-ci.org/TypeStrong/typedoc)
 [![NPM Version](https://badge.fury.io/js/typedoc.svg)](http://badge.fury.io/js/typedoc)
 [![Chat on Gitter](https://badges.gitter.im/TypeStrong/typedoc.svg)](https://gitter.im/TypeStrong/typedoc?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Greenkeeper Enabled](https://badges.greenkeeper.io/TypeStrong/typedoc.svg)](https://greenkeeper.io/)
 
 ## Installation