Include <sdf/sdf.hh> instead of <sdf/SDF.hh>

The sdformat package recently changed the name of an sdf header
file from SDF.hh to SDFImpl.hh; this change will use the lower-case
header file which should work with old and new versions of sdformat
or gazebo.
changelogs for 2.1.5
2.1.5
Merge branch 'hydro-devel' into merge_hydro_into_groovy