Ruby 2.1 workaround: Array#uniq does not always choose the first item

 * Ruby 2.1 workaround: Array#uniq does not always choose the first item.
   Thanks to Andrew Grimm who reported the issue.
   (https://github.com/bioruby/bioruby/issues/92 )
   Note that the behavior change is also regarded as a bug in Ruby and
   is fixed. (https://bugs.ruby-lang.org/issues/9340 )