devtools::install_github("imbs-hl/ranger") fails - make.exe and sh.exe unable to start correctly

I'm trying to install the development version of the R library ranger from RStudio on Windows 7 with the command:

devtools::install_github("imbs-hl/ranger")

When I try this, I first get a pop-up with the header "make.exe - Application Error" and the message "The application was unable to start correctly (0xc000007b)."

I then get a message saying that I need to install additional build tools. This downloads RTools. I then get an identical pop up like the make.exe one, except that this time it's for sh.exe. After I click ok, it then does a somewhat long install of RTools. The setup window closes and the development packages remain uninstalled.

Note: when I run either sh.exe or make.exe from the windows command line, I get the same errors. This might have something to do with the fact that I have cygwin installed.

I can get sh.exe and make.exe to run from the windows command line if I remove RTools from the path, leaving only cygwin. However, this doesn't change the problem I get when I try installing the development version of ranger.

Any help would be appreciated.