Specify bigint as test dependency

bigint is in core since 5.8 but Fedora Linux system perl ships without
it. This leads to cpan installers not having it and failing tests.

See
http://www.cpantesters.org/cpan/report/394d08da-047c-11e7-9c42-6ad91cc46bb4

Makefile.PL

@@ -475,7 +475,8 @@ if (eval $ExtUtils::MakeMaker::VERSION >= 5.43) {
         },
       },
     },
-    TEST_REQUIRES => { 'Test::Simple' => '0.90',
+    TEST_REQUIRES => { 'bigint'       => 0,
+                       'Test::Simple' => '0.90',
                        'Test::Deep'   => 0,
                        'Time::HiRes'  => 0,
     },