Depend on MXMS 0.21 to make sure type aliasing is working.

Makefile.PL

@@ -10,7 +10,7 @@ requires 'namespace::clean';
 requires 'namespace::autoclean' => '0.05';
 requires 'Moose' => '0.82';
 requires 'MooseX::AttributeHelpers';
-requires 'MooseX::Method::Signatures' => '0.19';
+requires 'MooseX::Method::Signatures' => '0.21';
 requires 'MooseX::Role::Parameterized' => '0.12';
 requires 'MooseX::Types::Moose';
 requires 'List::MoreUtils';