Old 0install can't run an alias created by a newer version

I'm mostly using the development ocaml version on my own machine. But
sometimes I end up running a released python (or even ocaml) version.
Unfortunately, this means I can't run any of my newer aliases, as I get:

Traceback (most recent call last):
  File "/home/tim/dev/0install/zi-stable/0install", line 14, in <module>
    main(sys.argv[1:])
  File "/home/tim/dev/0install/zi-stable/zeroinstall/cmd/__init__.py", line
381, in main
    cmd.handle(config, options, args)
  File "/home/tim/dev/0install/zi-stable/zeroinstall/cmd/run.py", line 39,
in handle
    r = app.get_requirements()
  File "/home/tim/dev/0install/zi-stable/zeroinstall/apps.py", line 389, in
get_requirements
    setattr(r, k, v)
AttributeError: 'Requirements' object has no attribute 'may_compile'


I guess this is my fault for running multiple versions of 0install, but
that's actually difficult to avoid sometimes. In some cases I end up inside
a shell (e.g 0env) which is making use of the old python ZeroInstall.xml
feed, so the python 0install ends up on my path. In other cases, I run the
released version of 0install (e.g on my mac laptop) but I have a single
~/.config/0install/ that I share between machines via a git repo.


I'm not sure what should be done about this. Some options:

1) only save requirement flags which are explicitly specified at creation
time, and rely on the defaults for other flags

2) ignore requirement keys which we don't understand, for forwards
compatibility.

(1) is probably harder, but (2) opens us up to silently ignoring important
requirements. So I'd vote for (1).

Cheers,
 - Tim.

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