Use File.delte instead of rm_r to avoid the following error at rubyci.org.
  ArgumentError: /home/chkbuild/build/ruby-1.9.3/20130115T073835Z/rubyspec/optional/capi/ext/constants_spec.so is not prefixed by /home/chkbuild/build/ruby-1.9.3/20130115T073835Z/rubyspec_temp
  /home/chkbuild/build/ruby-1.9.3/20130115T073835Z/rubyspec/optional/capi/spec_helper.rb:58:in `compile_extension'
This error occurs when specifying a different interpreter from the last one using mspec's -t option.
See http://u32.rubyci.org/~chkbuild/ruby-1.9.3/log/20130115T073835Z.log.html.gz.