--2.0 flag? Here is one way to check if you used jruby to install ruby-processing:-
Or to use built-in jruby-complete-1.7.4 you could set the JRUBY_OPTS environmental variable:-
jruby --2.0 -S rp5 sketch.rb
A third way is to set the compat.version in .jrubyrc (home directory or even the local directory).
At present ruby-2.0 support is experimental (and incomplete) with jruby-1.7.4, however it may even be default by jruby-1.7.5?
Anyway so far so good I have not found any problems with ruby-processing-1.0.13.