Attempt to fall back to install_git when install_github fails. 

This is useful on linux machines where libcurl is unavailable, and potentially
in other cases as well.

This patch addresses problems like those noted here:
https://github.com/ramnathv/rCharts/wiki/Installation---dependencies-errors-in-Linux-OS-(Ubuntu)

There is likely a more elegant way to avoid the problem than a big tryCatch, but it does have the advantage of not messing with anything unless errors are encountered. If you have any other ideas I'm willing to help test them out.