I tried using install_github and got the follow:
  library('devtools')
  > install_github('ramnath/slidify')
  Downloading github repo ramnath/slidify@master
  Error in loadNamespace(name) : there is no package called ‘httr’

I'm not sure why the namespace is a problem.
I've downloaded slidify from github to a zip file and tried installing it again, but it results in the same error.
thank you for the help.

I installed devtools 1.6.1 (and I installed Slidify afterwards independently just to make sure)  which are both listed in packages.  The error message seems to indicated the version of Slidify does not work with RStudio version 3.1.1, "package ‘slidify’ is not available (for R version 3.1.1". I cannot find a newer version of Slidify so I don't know if I need to revert back to an earlier version to get it to work or if I should just review the material via pdf files.

I have the same problem with R version 3.1.1. Here is the command line result of attempting to install it.
install.packages("slidify")
Installing package into ‘C:/Users/Patrick/Documents/R/win-library/3.1’
(as ‘lib’ is unspecified)
Warning in install.packages :
package ‘slidify’ is not available (for R version 3.1.1)


I cannot find a version that supports R version 3.1.x)