Remove theme_set() on startup and change message

[malcolmbarrett]

Closes #257

Reflects changes to bayesplot approach to themes in stan-dev/bayesplot#149

[jgabry]

Thanks for this! I’ll review the bayesplot one first and once that’s merged I’ll review/merge this one for rstanarm.

[jgabry]

And don’t worry about Travis-ci errors unless they occur before it times out. But it will probably time out and error even if all is ok, so we’ll have to check it locally.

[malcolmbarrett]

I'm going to update this to match the bayesplot startup message. This one is much simpler, but anything else that needs to be done before merging? I checked the vignettes that use raw ggplot2, but they seem to be setting the theme to theme_default in the background.

[malcolmbarrett]

Although, now that I say it, it makes less sense for the vignettes that have raw ggplot2 to be using theme_default() without explicitly calling it in the code since rstanarm will no longer change the theme for all ggplots.

[bgoodri]

@jgabry What is up with this PR?