Exclude json parsing of manifest value

As of 8d503a8e991888c5c87b256be03fe41ba7ec0248 it seems json manifest support is broken.
eg. `webassets.manifest = json:%(here)s/manifest.json`

http://webassets.readthedocs.org/en/latest/environment.html?highlight=manifest#webassets.env.Environment.manifest

This excludes the manifest value from being parsed as json.