Relax MakeMaker version restriction

It turned out that the version required for installation of this module was
too high and this caused problems for downstream modules when configuring
the module for installation.  This puts the version restriction back to that
matching when the TEST_REQUIRES parameter was added.

Makefile.PL

@@ -1,7 +1,7 @@
 use strict;
 use warnings;
 
-use ExtUtils::MakeMaker '7.0';
+use ExtUtils::MakeMaker '6.64';
 
 WriteMakefile(
     NAME            => 'Devel::LeakGuard::Object',