Fixes #5760: Check the Java version before trying to run Jetty

Ticket: http://www.rudder-project.org/redmine/issues/5760

To be merged in 3.0
Why use a wildcard character to match the version number that you're then going to compare to an int? If there is one place you need to use a character class in the regex, it's here! Please replace ```. *``` by the appropriate numerical pattern and self merge.