title: ggvis with shiny runtime produces inconsistent output on EC2
body: <p>I'm creating a configurable, interactive report with Rmarkdown, Shiny and ggvis. I develop on a Mac, then deploy to a Shiny server running Ubuntu on EC2. On EC2, my reactive ggvis plots fail to render, and instead just echo the reactive code:
  <a href="https://i.stack.imgur.com/GIfmL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GIfmL.png" alt="enter image description here"></a> </p>
  
  <p>Locally, I have no problems rendering the reactive plot:</p>
  
  <p><a href="https://i.stack.imgur.com/q4zMM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/q4zMM.png" alt="enter image description here"></a></p>
  
  <p>Has anyone seen this before? What's causing the inconsistent behavior? </p>
  
  <p>Self-contained example here:</p>
  
  <pre><code>---
  title: "test"
  runtime: shiny
  output: html_document
  ---
  
  ```{r config}
  require(ggvis)
  
  inputPanel(selectInput('dataset', 'Data Set:', c('one', 'the other')),
             actionButton('run', 'Run!'))
  
  data = eventReactive(input$run, {
    if (input$dataset == 'one') {
      data = data.frame(x = 1:20, y = rnorm(20))
    } else {
      data = data.frame(x = 1:20, y = rnorm(20, mean = 10, sd = 10))
    }
    return(data)
  })  
  ```
  
  ```{r plot}
  reactive({
    data() %&gt;%
      ggvis(x = ~x, y = ~y) %&gt;%
      layer_points(size := input_slider(min = 1, max = 100)) %&gt;%
      bind_shiny('plot', 'plot_ui')
  })
  
  uiOutput('plot_ui')
  ggvisOutput('plot')
  ```
  </code></pre>