Pin werkzeug version to prevent AttributeError: 'Request' object has no attribute 'is_xhr'

[greyli]

Originated from this issue in Flask.

The request.is_xhr property was deprecated since Werkzeug 0.13 and removed in Werkzeug 1.0.0. You will get this error when using Flask <= 0.12.4 and Werkzeug >=1.0.0 because Flask uses this property in the source before the 1.0.0 version.

This PR set the version of Werkzeug to 0.12.2 (the initial version in this project).