Require at least Graph.pm 0.50

Version 0.02 of Graph::MaxFlow failed a CPAN Testers test
(http://www.cpantesters.org/cpan/report/b0e354ae-0a9f-11e0-903e-ed941cbd8280)
because the tester was running an extremely old version of Graph.pm.  Turns out
that Jarkko completely changed the API for version 0.50, and I can't rely on
earlier versions to work.

Makefile.PL

@@ -7,7 +7,7 @@ WriteMakefile(
     (MM->can('signature_target') ? (SIGN => 1) : ()),
     'NAME'		=> 'Graph::MaxFlow',
     'LICENSE'           => 'perl',
-    'PREREQ_PM'         => { 'Graph' => 0 },
+    'PREREQ_PM'         => { 'Graph' => "0.5" },
     'VERSION_FROM'	=> 'MaxFlow.pm', # finds $VERSION
     'dist'              => { COMPRESS => 'gzip -9f', SUFFIX => 'gz', },
     ($] >= 5.005 ?    ## Add these new keywords supported since 5.005