Bump Akka version to 2.3.2

Quoth @rkuhn:

> seeing that you are cutting a Scala release, we’ll need to
> decide on which Akka version to bundle with it. 2.3.0 has
> some remoting bugs which we’d like to leave behind

Given the difficult of overriding the bundled Akka version
from the Scala script runner (SI-8472), we've deemed it
worthwhile to bump the Akka version before releasing 2.11.0.