Avoid RuboCop on RBX; it likes to SIGSEV in Travis

Author: tamird

Rakefile

@@ -8,13 +8,13 @@ load 'tasks/compile.rake'
 load 'tasks/generate.rake'
 load 'tasks/benchmarks.rake'
 
-# TODO: remove when we end support for < 1.9.3
-begin
+# TODO: remove engine check when rubinius stops crashing on RuboCop
+# TODO: remove defined?(Encoding) when we end support for < 1.9.3
+if defined?(RUBY_ENGINE) && RUBY_ENGINE == 'ruby' && defined?(Encoding)
   require 'rubocop/rake_task'
   RuboCop::RakeTask.new
   task :default => [:spec, :rubocop]
-
-rescue LoadError
+else
   warn 'RuboCop is not available'
   task :default => :spec
 end