Add check in build that getdown jars match the getdown src

The getdown-launcher.jar and getdown-core.jar found in getdown/lib (and also j8lib and j11lib) might not match the src version of getdown in getdown/src/getdown (since a rebuild of these jars is NOT part of the build system [currently a maven job by running cd getdown/src/getdown; ./mvn_cmd]).

Add a check in the build system that fails if the version don't match.