title: R version 3.1.2 on Windows 7 install.packages("devtools") fail
body: <p>I am unable to install devtools package using RStudio on Windows7 with errors below[*].
  Appreciated in advance for any help suggestions.</p>
  
  <pre><code>&gt; install.packages("devtools")
  Warning in install.packages :
    unable to resolve 'cran.rstudio.com'
  Warning in install.packages :
    unable to resolve 'cran.rstudio.com'
  Warning in install.packages :
    unable to access index for repository http://cran.rstudio.com/bin/windows/contrib/3.1
  Installing package into ‘C:/Users/Lucy/Documents/R/win-library/3.1’
  (as ‘lib’ is unspecified)
  Warning in install.packages :
    unable to resolve 'cran.rstudio.com'
  Warning in install.packages :
    unable to resolve 'cran.rstudio.com'
  Warning in install.packages :
    unable to access index for repository http://cran.rstudio.com/bin/windows/contrib/3.1
  Warning in install.packages :
    package ‘devtools’ is not available (for R version 3.1.2)
  </code></pre>