* Fix the following rake error since mruby-1.1.0.

  The error occurs when defining multiple configurations in build_config.rb.
  The build_config.rb in mruby-1.1.0 has two configurations ("host" and "host-debug").

~~~
rake aborted!
bad value for range
c:/dev/mruby/build/mrbgems/mruby-require/mrbgem.rake:52:in `block (2 levels) in <top (required)>'
c:/dev/mruby/tasks/mruby_build.rake:12:in `instance_eval'
c:/dev/mruby/tasks/mruby_build.rake:12:in `block in each_target'
c:/dev/mruby/tasks/mruby_build.rake:11:in `each'
c:/dev/mruby/tasks/mruby_build.rake:11:in `each_target'
c:/dev/mruby/build/mrbgems/mruby-require/mrbgem.rake:45:in `block in <top (required)>'
c:/dev/mruby/tasks/mrbgem_spec.rake:80:in `instance_eval'
c:/dev/mruby/tasks/mrbgem_spec.rake:80:in `setup'
c:/dev/mruby/tasks/ruby_ext.rake:41:in `block in to_proc'
c:/dev/mruby/tasks/mrbgem_spec.rake:286:in `each'
c:/dev/mruby/tasks/mrbgem_spec.rake:286:in `each'
c:/dev/mruby/tasks/mrbgems.rake:4:in `block in <top (required)>'
c:/dev/mruby/tasks/mruby_build.rake:12:in `instance_eval'
c:/dev/mruby/tasks/mruby_build.rake:12:in `block in each_target'
c:/dev/mruby/tasks/mruby_build.rake:11:in `each'
c:/dev/mruby/tasks/mruby_build.rake:11:in `each_target'
c:/dev/mruby/tasks/mrbgems.rake:1:in `<top (required)>'
c:/dev/mruby/Rakefile:26:in `load'
c:/dev/mruby/Rakefile:26:in `<top (required)>'
(See full trace by running task with --trace)
~~~