Add XWiki snapshot maven repository by default for snapshot builds

It is more and more of a burden (and a source for false-positive errors) to have to manually modify xwiki.preferences each time you want to try out a jetty-mysql snapshot (specifically for XEM).

Since the default maven repo is provided by the (maven) repository implementation, we can`t do it in xwiki.preferences.vm unless we uncomment the defaults. If we don`t uncomment the default and just add the snapshots repo, the other repos (defualt ones) will not be available (since they are commented). Thus, I guess we could check the xwiki version at runtime, in the maven repository implementation, and, if it ends(or contains) SNAPSHOT, we can add the snapshots maven repository to the list.