Add demjson to requirements

Demjson is a library used by openstack-doc-tools for checking JSON
syntax. Its error messages are much more useful than the terse messages
provided by the standard json library.

For a unified diff of output with and without demjson (checking the same
set of JSON sample files), see:

http://paste.openstack.org/show/96358/

Change-Id: I2b353f3cf04f4e233d18dcda1160f50316c337d8
Merge "Add demjson to requirements"