don't include Shutterbug in gh-pages branch

The gh-pages branch is delivered on a static file-system
so shutterbug/shutterbug.js (which is delivered by the
Rack application Shutterbug) is not available.

Use two SHUTTERBUG related partials to conditionally
include the request for: shutterbug/shutterbug.js

FIXME:
This should also be done when generating the versioned
tar.gz archives … but since they are created from running
code on a server using Rack, Shutterbug can't be easily
removed ...