Let bargap adapt and fix x-labels in category histograms

This PR fixes category histograms (bar gap and x-labels).
Before: https://plot.ly/~marianne2/98
After: https://plot.ly/~marianne2/96

* Image diffs can be seen here: https://github.com/ropensci/plotly/commit/89870edbef0599b8f17b31d6af4b480cdedbf6fe
Look for JSON changes (i.e., `test-ggplot-histogram-barmodestack.json`, `test-ggplot-histogram-binwidth.json`, `test-ggplot-histogram-dates.json`) and corresponding image files, as @chriddyp suggested.

I'll increment the package version number after reviews.

/cc @pedrodz