dak_utils.build_file_list does not like non-float Formats

dak_utils.build_file_list assumes that the Format field in a .dsc can be float()ed. This is no longer a valid assumption, with formats '3.0 (quilt)' and '3.0 (native)' popping up, in addition to the original '1.0'.

This results in sync-source.py crashing like this:

Traceback (most recent call last):
  File "../scripts/ftpmaster-tools/sync-source.py", line 911, in ?
    main()
  File "../scripts/ftpmaster-tools/sync-source.py", line 907, in main
    do_diff(Sources, Suite, origin, arguments, current_binaries)
  File "../scripts/ftpmaster-tools/sync-source.py", line 696, in do_diff
    Options.requestor, origin, Suite, current_binaries)
  File "../scripts/ftpmaster-tools/sync-source.py", line 644, in add_source
    current_binaries)
  File "../scripts/ftpmaster-tools/sync-source.py", line 369, in import_dsc
    dsc_files = dak_utils.build_file_list(dsc, is_a_dsc=1)\
  File "/home/wgrant/launchpad/lp-branches/devel/scripts/ftpmaster-tools/dak_utils.py", line 157, in build_file_list
    format = float(format);
ValueError: invalid literal for float(): 3.0 (quilt)