define dependency versions in build.xml to be easily overridden in build.properties

Dependency versions are defined in ivy.xml, which is suboptimal since it is hard to override them from a script.

If we defined the versions in the main build.xml (just as we do with audience-annotations.version) and use variables in ivy.xml then we could easily override the versions with creating a build.properties file, which mechanism is already built in.
This way the dependency versions could be replaced by sed or any simple command line tool.