Avoid Array#select!

Array#select! was added in ruby 1.9.2.  Using it limits users to Ruby
1.9.2 or newer.  To maximize compatibility with as many versions og Ruby
as possible, use of Array.select! should be avoided.  FWIW, this problem
was discovered because Ubuntu 10.04 LTS (which is rather old now)
provides Ruby 1.9.1.