#104 issue fix, search InstallPath in SOFTWARE\R-core\R\<ver> keys

Issue: https://github.com/BlueMountainCapital/FSharpRProvider/issues/104#issuecomment-67295196

On my machine (Windows 7 x64, R 3.1.0) in registry HKEY_LOCAL_MACHINE\SOFTWARE\R-core\R no  value 'InstallPath', but HKEY_LOCAL_MACHINE\SOFTWARE\R-core\R\3.1.0 contains it.

![r-no-installpath](https://cloud.githubusercontent.com/assets/428187/5506166/8aeec56e-87a8-11e4-85d6-623d27be36d5.png)
![r-3 1 0-has-installpath](https://cloud.githubusercontent.com/assets/428187/5506167/8ce9d57a-87a8-11e4-95cc-62900d42a530.png)