disable stmbench7 building

Summary: The ScalaSTM target for stmbench7 relied on the internal
implementation of Scala's immutable trees to extend the provided
functionality, but those have been changed in Scala 2.11.  This diff
moves them out of the set of build targets.
Default target is now 2.11.2
AtomicArray.ofLong truncates values to 4 byte width during read

Issue #42