mruby-bin-mruby: Add test dependency.

`Kernel#p` etc are used.

mrbgems/mruby-bin-mruby/mrbgem.rake

@@ -5,6 +5,7 @@ MRuby::Gem::Specification.new('mruby-bin-mruby') do |spec|
   spec.bins = %w(mruby)
   spec.add_dependency('mruby-compiler', :core => 'mruby-compiler')
   spec.add_dependency('mruby-error', :core => 'mruby-error')
+  spec.add_test_dependency('mruby-print', :core => 'mruby-print')
 
   if build.cxx_exception_enabled?
     build.compile_as_cxx("#{spec.dir}/tools/mruby/mruby.c", "#{spec.build_dir}/tools/mruby/mruby.cxx")