prevent conflict with version set in lib/Moonpig/UserAgent.pm

Without this change, `dzil listdeps` fails with:

    [@Filter/PkgVersion] existing assignment to $VERSION in lib/Moonpig/UserAgent.pm
    [@Filter/PkgVersion] existing assignment to $VERSION in lib/Moonpig/UserAgent.pm at /home/mjd/lib/perl/lib/perl5/x86_64-linux/Moose/Meta/Method/Delegation.pm line 110.

and `dzil test` fails similarly.