title: devtools::install_github() fails on map drive or VM-based OS
body: <p>I am trying to install <a href="https://github.com/Espanta/lubripack" rel="nofollow noreferrer">this</a> package from github using <code>install_github("espanta/lubripack")</code>. When I run from a stand-alone machine (not a VM), it works fine and package gets installed easily. However, when I run the command on a OS that runs as a VM, it fails to install by throwing below error</p>
  
  <pre><code>Downloading GitHub repo espanta/lubripack@master
  from URL https://api.github.com/repos/espanta/lubripack/zipball/master
  Installing lubripack
  "C:/PROGRA~1/R/R-32~1.3/bin/x64/R" --no-site-file --no-environ --no-save --no-restore CMD INSTALL  \
    "C:/Users/Espanta/AppData/Local/Temp/RtmpsRvcqY/devtools19007474782/Espanta-lubripack-b1dd9ee"  \
    --library="\\psf/Home/Documents/R/win-library/3.2" --install-tests 
  
  * installing *source* package 'lubripack' ...
  ** R
  ** preparing package for lazy loading
  ** help
  *** installing help indices
  ** building package indices
  ** testing if installed package can be loaded
  *** arch - i386
  Warning in library(pkg_name, lib.loc = lib, character.only = TRUE, logical.return = TRUE) :
    there is no package called 'lubripack'
  Error: loading failed
  Execution halted
  *** arch - x64
  Warning in library(pkg_name, lib.loc = lib, character.only = TRUE, logical.return = TRUE) :
    there is no package called 'lubripack'
  Error: loading failed
  Execution halted
  ERROR: loading failed for 'i386', 'x64'
  * removing '\\psf/Home/Documents/R/win-library/3.2/lubripack'
  Error: Command failed (1)
  </code></pre>
  
  <p>I run <code>Parallel Desktop</code> on my MAC to host Windows 7 OS. So the windows is installed on a network-like partition <code>(i.e., \\psf)</code>. But it fails installation.</p>
  
  <p>Similar discussion was made <a href="https://stackoverflow.com/questions/14755033/r-custom-package-install-from-file-error">here</a> in another stackexchage thread, but in a slightly different context. Although in the 4th line of the above output the windows-like address is also provided <code>"C:/PROGRA~1/R/R-32~1.3/bin/x64/R"</code>, it seems something still goes wrong. I think I should be able to explicitly give the address to install_github() function so it can identify the location. Any help is appreciated especially from Hadley Wickham who is the creator and owner of the Devtools package.</p>