Manage module versions via parent pom

Put module versions in dependency management of parent
pom. Also moved to version 15.0 of guava because that
is what jackrabbit-oak is using too.
Use streams for getting values

For consistency I now get the values of a property as
a stream instead of a list.
Fix compile errors.

After java 8 upgrade I got compile errors that I did
not have with the previous java 8 version. Don't know
why, but this change fixes the compile errors.