Update preprocessed b-tag SF branch names

With https://github.com/cms-nanoAOD/nanoAOD-tools/pull/233 the names of the branches for the b-tagging scale factors in preprocessed nanoAODs has changed. This updates the decorators to reflect those changes.

Note: it's just a quick fix for now but making it configurable would be useful indeed... Would it be an option to pass a getJetMETVarName_postproc function as an argument to prepareTree(), with as default the current implementation, or did you have something else in mind?