updated github.io bistro url
updated github.io bistro url

Summary:
Updated bistro URL http://facebook.github.io/bistro/
Closes https://github.com/facebook/bistro/pull/26

Differential Revision: D7230829

Pulled By: snarkmaster

fbshipit-source-id: 50c45cbcfa8b70730b1058d87034954d817d06c4

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro