Fixes intermittent ConcurrencyError seen in JRuby

This fixes a ConcurrencyError seen intermittently in JRuby 1.7.4 where a separate native thread was writing to stdout.  This error was observed in v1.3.8.  

>Detected invalid array contents due to unsynchronized modifications with concurrent users (ConcurrencyError)
org/jruby/RubyArray.java:1153:in <<' /home/jenkins/.rvm/gems/jruby-1.7.4/gems/cucumber1.3.8/bin/../lib/cucumber/formatter/interceptor.rb:14:inwrite'
org/jruby/RubyIO.java:2350:in write' org/jruby/RubyIO.java:2305:inputs'
org/jruby/RubyKernel.java:577

This may be an issue for other non-MRI versions of ruby as well.