Convert violin plot as boxplot for now

A violin plot is a combination of a box plot and a kernel density plot (Wikipedia).

Convert the box plot part for now, so that calling `ggplotly()` on a violin plot does not return an error, but just a warning.

As kernel density estimations (KDE) become available in Plotly, we will support more faithful conversions.

/cc @xsaintmleux @pedrodz @chriddyp