Update OSX Java 1.6 compatibility fix for plugins

Change the initial fix for OSX Java 1.6 compatibility by removing the
need for a direct `tools.jar` dependency.

It appears that `system` dependencies are always pulled in when
used in a Maven plugin. This makes the the dependency on `tools.jar`
particularly brittle since we need to make assumptions about where
the jar is located.

Since the tools jar is *only* needed for Spring Loaded support, the
plugin has now been updated to locate the jar programmatically and
call it via reflection. This should reduce the risk of problems when
using the plugin and at the very least isolate them to Spring Loaded
support.

(See original commit b8c4720)

Fixes gh-497