update pom to Prism 2.0 beta6 per recommendation of viveleroi that it should solve some of the stack traces seen in the 1.6 version.

Turns out the Prism API hasn't changed so Heimdall built fine with no code changes needed. So this change doesn't do anything except update the POM to reflect that Heimdall is now building and working against Prism 2.0 as well.