Explicitly require EU::MM 6.64

TEST_REQUIRES makes its debug in EU::MM 6.64, and without it,
the proper version of Test::More may not be installed and the tests will
fail (ex. on perl 5.8.9)  For example:

   perlbrew install --as 5.8 5.8.9
   perlbrew use 5.8
   cpanm Module::Metadata # this fails