install_github() tries to install deps in the wrong lib dir

[dtenenba]

Related to #869, if I am trying to install a package that has a dependency on rmarkdown and rmarkdown is not installed, I see this:

> install_github("dtenenba/anRpackage", build_vignettes=TRUE,   repos="http://bioconductor.org/packages/3.2/bioc",   dependencies=TRUE, type="source")
Downloading github repo dtenenba/anRpackage@master
Installing anRpackage
Skipping 1 packages not available: BiocCheck
Installing 1 packages: rmarkdown
Warning in utils::install.packages(pkgs, repos = repos, type = type, ...,  :
  'lib = "http://cran.rstudio.com"' is not writable
Would you like to use a personal library instead?  (y/n) 

Sounds like devtools (or someone?) thinks that it should install the package in the http://cran.rstudio.com directory on my machine, which is obviously not a valid directory name.
Not sure how the lib argument got set to that. I have no trouble installing the package with:

install.packages("rmarkdown", repos="http://cran.rstudio.com")

> session_info()
Session info -------------------------------------------------------------------
 setting  value                       
 version  R version 3.2.1 (2015-06-18)
 system   x86_64, darwin13.4.0        
 ui       X11                         
 language (EN)                        
 collate  en_US.UTF-8                 
 tz       America/Los_Angeles         

Packages -----------------------------------------------------------------------
 package   * version date       source        
 curl        0.9.1   2015-07-04 CRAN (R 3.2.0)
 devtools  * 1.8.0   2015-05-09 CRAN (R 3.2.0)
 digest      0.6.8   2014-12-31 CRAN (R 3.2.0)
 git2r       0.10.1  2015-05-07 CRAN (R 3.2.0)
 memoise     0.2.1   2014-04-22 CRAN (R 3.2.0)
 Rcpp        0.11.6  2015-05-01 CRAN (R 3.2.0)
 rversions   1.0.1   2015-06-06 CRAN (R 3.2.0)
 xml2        0.1.1   2015-06-02 CRAN (R 3.2.0)

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