Question: I'm trying to get `devtools::install_github()` working behind my corporate proxy on Windows 7. 

So far I've had to do the following:

```
> library(httr)
> library(devtools)
> set_config(use_proxy("123.123.123.123",8080))
> devtools::install_github("rstudio/ggvis")

Installing github repo ggvis/master from rstudio
Downloading master.zip from https://github.com/rstudio/ggvis/archive/master.zip
Error in function (type, msg, asError = TRUE)  : 
  SSL certificate problem, verify that the CA cert is OK. Details:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

```

Apparently we have some kind of certificate server replacing SSL certs with our own corporate SSL certs (confirmed by going to <https://github.com> and examining the cert). 

Anyhow, just wondering if there's a way to ignore that cert error and proceed with the installation?

Answer: 
One way to handle the problem is to set the `CURLOPT_SSL_VERIFYPEER` to false. This option determines whether curl verifies the authenticity of the peer's certificate. A value of 1 means curl verifies; 0 (zero) means it doesn't.
<http://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYPEER.html>

The relevant option needs to be passed to `RCurl`. In `RCurl` the `CURLOPT_` is removed letters arre lowercase and the underscore is changed to `.`. 

```
set_config( config( ssl.verifypeer = 0L ) )

```

will pass the relevant option to `RCurl` when using `httr`.

UPDATE:

The `httr` since this answer was written has moved from RCurl as an underlying dependence to the `curl` package. cURL options are now specified 
with underscores so the above would be:

```
set_config( config( ssl_verifypeer = 0L ) )

```

in the current version of `httr`.
I tired @jdharrisonbut method but it didn't worked for me. 

A Quick fix to avoid using https connection, is by replacing https by http. you install a package from cran by this method.

install.packages("<http://cran.r-project.org/src/contrib/ggvis_0.4.4.tar.gz>", repo=NULL, type="source")