Update plugin version in scripted tests to 1.6.0-SNAPSHOT.
Fix filtering of core libraries when using SBT 0.13.0 and Scala != 2.10.2

Before 0.13.0, we *always* used to get JARs from the boot directory; now we
only seem to get these if we use the same Scala version as SBT itself (2.10.2).

This commit relaxes the filtering to handle both naming conventions.