Correct the detection and guide for JRuby coverage
Correct the detection and guide for JRuby coverage

`JRuby.runtime.debug?` does not answer correctly if coverage is active for JRuby.  `org.jruby.RubyInstanceConfig.FULL_TRACE_ENABLED` does.

Enabling general debug mode on JRuby does not enable coverage.  You need the "--debug" flag on the command line or in JRUBY_OPTS, or set the "debug.fullTrace=true" option in ".jrubyrc".

This change should make using simplecov on JRuby a bit nicer.
colszowka/simplecov,colszowka/simplecov