Updated manifest.xml and package.xml for autoproj/rosdep compatibility

This pull request tracks manifest changes in utilrb for autoproj/rosdep compatibility and for consistency between `manifest.xml` and `package.xml`. See discussion started in https://github.com/orocos-toolchain/utilrb/issues/5.
See also @doudou's comments on the outdated commit https://github.com/meyerj/utilrb/commit/6a156fd71244452de539fb514a8ae213e5464ad5.