STDOUT redirection is not thread-safe on JRuby and Rubinius

[afn]

The fix introduced by #855 addresses MRI but not JRuby or Rubinius because of incomplete support for the :out parameter to Process.spawn on those platforms.

Apparently newer versions of JRuby do support :out. We should determine which version of JRuby fixed this (and from there, determine whether we care about older versions of JRuby). We should also explore whether the issue is OS-specific.