title: Adding ggvis plot in RMarkdown document makes knitr::kable output render incorrectly
body: <p>Reproducible example below. I lose formatting on the table whenever I include a <code>ggvis</code> figure.</p>
  
  <pre><code>---
  title: "test"
  output: html_document
  ---
  
  ```{r setup, include=FALSE}
  library(dplyr)
  library(ggvis)
  library(knitr)
  ```
  
  The following table looks fine...
  
  ```{r echo=FALSE, results='asis'}
  cars %&gt;% kable(format = 'markdown')
  ```
  
  As long as I don't include this plot below
  
  ```{r, echo=FALSE}
  pressure %&gt;%
    ggvis(x = ~temperature, y = ~pressure) %&gt;%
    layer_bars()
  ```
  </code></pre>