Allow overriding akka-actors.version from parameter.

This allows setting a new akka-actors using a jenkins parameter.

The commit also fixes a problem when SCALA_VER_BASE is defined.
In this case, versions.properties was not parsed, and
deriveModuleVersions would not work as expected: we still want to
read module versions from versions.properties.
Pass updated scalacheck.version.number and jline.version to the core build

Just like AKKA_ACTOR_VER, JLINE_VER can be provided to use a new jline
release.

For all modules, the upgraded version number is also forwareded to
the core build if the new module version already existed. This way
we can upgrade versions.properties to a new module release that
is already published.
Merge pull request #119 from lrytz/master

Allow overriding akka-actors.version from parameter.