Sync jenkins dependency with git plugin

My first attempt to write a credentials test failed because Jenkins 1.480 would crash the trivial test at runtime.  When I switched to Jenkins 1.509, the test passed.  Since the git-plugin is already based on 1.509 and the primary consumer of git-client-plugin is the git-plugin, I think we should update the based Jenkins version of the git-client-plugin from 1.480 to 1.509.

The second change removes the JDK 1.5 property, since Jenkins now requires at least JDK 1.6 for Jenkins core.