title: Error message about username when using plotly r package offline
body: <p>I'm trying to begin using plotly for R, but I'm having trouble with an error about username. When I try to execute the following code... </p>
  
  <pre><code>test=ggplot(diamonds,aes(x=diamonds$carat,y=diamonds$price))+geom_point()
  test2=plotly(test)
  plotly(test)
  </code></pre>
  
  <p>...I get the error message: </p>
  
  <pre><code>Storing 'username' as the environment variable 'plotly_username'
  Error in Sys.setenv(plotly_username = username) : 
    wrong length for argument
  </code></pre>
  
  <p>I thought plotly's R package was available for use without a username via htmlwidgets. As far as I can tell, I'm using the latest version of plotly, ggplot, and htmlwidgets. What am I doing wrong? </p>