Set `canRetransform` flag to `false` in instrumentation.
Set `canRetransform` flag to `false` in instrumentation.

We do not need to retransform classes once they are loaded.
All instrumentation byte-code is pushed at loading time.

This fixes a problem with Java 7 that was failing to add
a transformer because we did not declare retransformation
capability in `MANIFEST.MF` file in Java agent jar.
Java 6 allowed to add transformer due to a bug.

som-snytt/scala-partest,scala/scala-partest,scala/scala-partest,SethTisue/scala-partest,lrytz/scala-partest