POD test breaks on old perls

[mauke]

See e.g. https://www.cpantesters.org/cpan/report/d648c9fc-2692-11f1-a3bc-a055f9c4ba34:

#   Failed test 'RT#53557: Date::Parse.pm has valid POD (no missing =back)'
#   at t/pod-valid.t line 16.
#          got: '1'
#     expected: '0'
# Looks like you failed 1 test of 1.
t/pod-valid.t .......... 
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/1 subtests 

This is because the test blindly accepts any Pod::Checker, but the one bundled with perl 5.8 doesn't understand =encoding yet.

Consider making this an author-only test.