docs: there are more than 50 contributors now (#703)

Github reports 105 on the project homepage, details: 
https://github.com/python-zk/kazoo/graphs/contributors

README.md

@@ -23,4 +23,4 @@ Project](http://www.nimbusproject.org/) and through collaboration with
 the open-source community has been merged with code from
 [Mozilla](http://www.mozilla.org/) and the [Zope
 Corporation](http://zope.com/). It has since gathered an active
-community of over fifty contributors.
+community of over one hundred contributors.