Use `Import-Package` instead of `Require-Bundle` for org.eclipse.core.runtime

This should allow Luna installs without conflicts due to “double vision” of
the `javax.bind.ws` package. This workaround was suggested in this Eclipse ticket.

https://bugs.eclipse.org/bugs/show_bug.cgi?id=430458

PS. Even though this is primarily a Luna concern, this works well for Kepler, so I prefer to keep