Fixing http://issues.roundup-tracker.org/issue2550859 which reports a
404 error accessing spec.html reference in the developer.html page.

Changed references to spec.html in developers.txt and installation.txt
to point to design.html since there is no spec.html.

I did track down a copy of what I think is supposed to be spec.html
from google cache of:

 http://zesty.ca/sc-roundup.html. It is a snapshot of the page as it
 appeared on Dec 4, 2014 22:56:45 GMT

obtained via:

http://webcache.googleusercontent.com/search?q=cache:Zyw8ZjphH2AJ:zesty.ca/sc-roundup.html

The copy of spec.html is missing images (www.lfw.org which hosted the
images seems to be gone).

I left the reference to spec.html in design.txt (aka design.html)
rather than having it point to itself. Also the text surrounding the
reference to spec.html discussses rationale and things that are
misssing from design.html.

I am not sure what can be done to retrieve the real spec.html with
images etc, or exactly how to handle the spec.html reference in
design.html.