Apply @masuidrive's review point. @bins should be Array.

tasks/mrbgem_spec.rake

@@ -29,7 +29,6 @@ module MRuby
       attr_accessor :test_preload
 
       attr_accessor :bins
-      alias :bin= :bins=
 
       attr_block MRuby::Build::COMMANDS
 
@@ -81,6 +80,10 @@ module MRuby
         define_gem_init_builder
       end
 
+      def self.bin=(bin)
+        @bins = [bin].flatten
+      end
+
       def build_dir
         "#{build.build_dir}/mrbgems/#{name}"
       end