title: R leaflet package produces blank (grey) map [Ubuntu 14.04]
body: <p>I tried out the examplecode from the <code>leaflet</code> package doc:</p>
  
  <pre><code>devtools::install_github("rstudio/leaflet");library(leaflet)
  m &lt;- leaflet() %&gt;%
    addTiles() %&gt;%  # Add default OpenStreetMap map tiles
    addMarkers(lng=174.768, lat=-36.852,
               popup="The birthplace of R")
  m  # Print the map
  </code></pre>
  
  <p>but it only produced a <strong>grey, blank map</strong> in the RStudio Viewer, as well as in my Firefox 38.0. Maybe this has something to do with Ubuntu?</p>
  
  <p>Here my sessionInfo:</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.2.1 (2015-06-18)
  Platform: x86_64-pc-linux-gnu (64-bit)
  Running under: Ubuntu 14.04.2 LTS
  
  locale:
  [1] LC_CTYPE=de_DE.UTF-8 LC_NUMERIC=C  C_TIME=de_DE.UTF-8   
      C_COLLATE=de_DE.UTF-8     LC_MONETARY=de_DE.UTF-8   
  [6] LC_MESSAGES=de_DE.UTF-8    LC_PAPER=de_DE.UTF-8 
   LC_NAME=C  LC_ADDRESS=C     LC_TELEPHONE=C           
  [11] LC_MEASUREMENT=de_DE.UTF-8 LC_IDENTIFICATION=C       
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] leaflet_1.0.0.9999 rgdal_0.9-2        sp_1.0-17         
  
  loaded via a namespace (and not attached):
  [1] Rcpp_0.11.5        magrittr_1.5       devtools_1.7.0
  MASS_7.3-41        munsell_0.4.2      colorspace_1.2-6   
  lattice_0.20-31   
  [8] stringr_0.6.2      httr_0.6.1         plyr_1.8.1  
  tools_3.2.1        grid_3.2.1         gtable_0.1.2       
  tmltools_0.2.6   [15] yaml_2.1.13        digest_0.6.8        
  JSONIO_1.3-0      RColorBrewer_1.1-2 reshape2_1.4.1     
  gplot2_1.0.1      htmlwidgets_0.3.2 
  [22] bitops_1.0-6       RCurl_1.95-4.5     scales_0.2.5   
  proto_0.3-10      
  </code></pre>