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