Remove Bokeh and Holoviews viewers for histograms (cruft: we'll probably use Vega or go indirectly through pandhist; definitely don't want BokehCanvas, which is a buggy version of [VegaScope](https://github.com/diana-hep/vegascope)).
2.8.13
Fixed bug 70, allowing empty ROOT files to be opened without raising exceptions.
2.8.12
Added Windows to continuous integration with [AppVeyor](https://ci.appveyor.com/project/jpivarski/uproot) and fixed an error with loading histograms.
2.8.11
Fixes 64: `TDirectoryFile` should be treated the same way as `TDirectory`. Files made with `TDirectoryFile` now work.
2.8.10
Fixed a small but crucial bug in calculation of TLorentzVector mass.
2.8.9
Fixes 63 by adding another way to recognize `std::string` branches.