Update CSS for better combination with revealjs

[atusy]

Regarding to rstudio/revealjs#57 , CSS in some frameworks possibly cause messy results on leaflet tiles.
This PR fixes it.

Here is an example messy result with revealjs (see reprex section for code).
Although @kazutan made PR on revealjs, I think making PR on leaflet fits better.

I did not add tests or update documentation because my change is limited to css.

reprex

Render an Rmd file with a following content.

---
output: revealjs::revealjs_presentation
---

```{r, echo=FALSE}
library(leaflet)
leaflet() %>% addTiles()
```

PR task list:

• Update NEWS
• Add tests (where appropriate)
  • R code tests: tests/testthat/
  • Visual tests: R/zzz_viztest.R
• Update documentation with devtools::document()

[CLAassistant]

All committers have signed the CLA.

[ElenaDatak]

using CSS you can manage it :

.reveal section img {  
  border: 0;
  box-shadow: 0;
}