Fix warning in t/pmc/bignum.t for GMP 5
Fix warning in t/pmc/bignum.t for GMP 5

This code was trying to warn about GMP prior to 4.1.4, but instead was
warning about any version before x.1.4.  I rearranged the logic to
make the intent more clear and to not warn about GMP 5+.

FROGGS/parrot,FROGGS/parrot,tkob/parrot,tkob/parrot,youprofit/parrot,tkob/parrot,tkob/parrot,tkob/parrot,parrot/parrot,youprofit/parrot,FROGGS/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,FROGGS/parrot,youprofit/parrot,parrot/parrot,youprofit/parrot,parrot/parrot,FROGGS/parrot,tkob/parrot,youprofit/parrot,FROGGS/parrot,tkob/parrot,youprofit/parrot,FROGGS/parrot,FROGGS/parrot,parrot/parrot,parrot/parrot