PowerShell 4.0 install failing on clean Windows 2008 R2 install.

Chef run will fail with "Chef::Exceptions::Win32RegKeyMissing" if trying to install PowerShell 4.0 on a node without PowerShell 3.0 as the registry key's will not exist (Windows 2008 R2 clean install). This change rescues that event and allows PowerShell 4.0 to install.