add dependency from mruby-string-utf8 to mruby-string-ext to ensure loading order

mrbgems/mruby-string-utf8/mrbgem.rake

@@ -2,4 +2,5 @@ MRuby::Gem::Specification.new('mruby-string-utf8') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
   spec.summary = 'UTF-8 support in String class'
+  spec.add_dependency('mruby-string-ext', :core => 'mruby-string-ext')
 end

mrbgems/mruby-string-utf8/test/string.rb

@@ -67,6 +67,6 @@ assert('String#rindex') do
   assert_equal 3, str.rindex('ち', 10)
 end
 
-assert('String#chr') do
+assert('String#chr(utf-8)') do
   assert_equal "こ", "こんにちは世界!".chr
 end