Deprecations in ggplot2 and tidyselect

[jgabry]

• .data used in tidyselect expressions is deprecated (fix depends on situation)
• aes_ is deprecated in ggplot2 (use tidy evaluation ideoms with aes() instead)
• for lines/borders/etc. size is now linewidth in ggplot2. For now in bayesplot we can use linewidth internally but keep any user facing arguments called size. Eventually we can decide if we want to add user facing linewidth arguments and deprecate any size arguments we have (I'm not sure how many of those there are off the top of my head) .

I have a PR (#292) that fixes these issues.