Bug fix: rejected candidates not always reported correctly

Initialising the first record field had the side-effect of populating
a mutable list used for the second field. But OCaml is free to
initialise the fields in any order (and it seems that the byte-code
does it backwards).
Removed python-gobject from the dependencies in the README
When building byte-code, we need -custom to include the C code
Don't include main 0install code in GTK plugin

In the byte-code, it registers with its copy of the GUI instead of with
the real one.
Release 2.6.1
Start development series 2.6.1-post
Merge branch 'b2.6'
Changed RPM cache file name

Avoids conflicts with older versions of 0install, which might write
corrupted files.

Reported by Tim Cuthbertson.
Cap distribution version number components to 9999999999999999

Prevents them wrapping around (very large numbers are usually hashes anyway).