Don't use experimental given/when construct

Since Perl 5.18, using the experimental switch construct (given/when) will
now trigger warnings like:

    when is experimental at .../blib/lib/File/XDG.pm line 93.

This can be reproduced by running the test suite on Perl 5.18 and later.
Merge pull request #8 from olof/topic/given_when

Don't use experimental given/when construct