Lower minVersion to 21 for Pale Moon compat.
Merge pull request #448 from jsha/4.0

Lower minVersion to 21 for Pale Moon compat.
Make install.rdf compatible with AMO.

AMO is picky about the values of minVersion/maxVersion fields, they have to be
listed in:
https://addons.mozilla.org/en-US/firefox/pages/appversions/

Note: In order to upload to AMO we'll also need to remove the updateURL and
updateKey fields.
install.rdf -> latest versions.
Merge pull request #456 from jsha/amo-install-rdf

Make install.rdf compatible with AMO.
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