[DF] Fix compilation error on mac1012, XCode9

Link to compilation error:
https://epsft-jenkins.cern.ch/view/ROOT%20Nightly/job/root-nightly-master/1977/LABEL=mac1012,SPEC=noimt/parsed_console/

The error is due to a bug in clang3.5, reproducible in isolation:
https://godbolt.org/z/jtYXv6

The workaround is to avoid specifying the template parameters of a
template base class -- the full type is injected in the derived class
anyway (thanks Axel!).