Re-enable CPU sampling on JRuby 1.7.0 and above

The problem with Process.times reporting wall clock elapsed time instead of CPU time has been fixed in JRuby 1.7.0 release. CPU sampling can therefore be enabled again for JRuby 1.7.0 and above.

See links below for reference:
http://jira.codehaus.org/browse/JRUBY-6089
https://github.com/jruby/jruby/commit/d49f05ea6f3c0a21426f6a2f9a81ce04805f7963#src/org/jruby/RubyProcess.java

The article below should also be updated when this commit becomes part of the gem.
https://newrelic.com/docs/ruby/new-relic-on-jruby