[ML] MeshQuality/RadiusEdgeRatioMetric: Remove unused variable.
Merge pull request #460 from TomFischer/FixWarning

[ML] Remove unused variable in RadiusEdgeRatioMetric.
Update for changed interface of boost >= 1.55.

The boost::property_tree::xml_writer_settings interface changed from Char type to String type. A preprocessor if for boost version <= 1.55 added to include old versions.