Revert to Debian 6 and Java 6 due to Ubuntu bug #874505.

The bug prevents multistrap from installing the newer distros properly
https://bugs.launchpad.net/ubuntu/+source/base-files/+bug/874505
This is possibly related or a duplicate:
https://bugs.launchpad.net/ubuntu/+source/base-files/+bug/942940
Use Oracle's JDK 7u11 with Debian 6 (Squeeze).