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
Merge pull request #751 from dragos/issue/resolution-order-luna

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