Enable CPU sampling on JRuby 1.7.0 and higher

The problem with Process.times reporting wall clock elapsed time instead of CPU time has been fixed in JRuby 1.7.0 release.

See links below for reference:
http://jira.codehaus.org/browse/JRUBY-6089
https://github.com/jruby/jruby/commit/d49f05ea6f3c0a21426f6a2f9a81ce04805f7963#src/org/jruby/RubyProcess.java