Catch exception when used on FullCLR

This is the case when the module is used on FullCLR versions of
PowerShell (necessarily on Windows).

PowerShellGitHubDev.psm1

@@ -1,9 +1,15 @@
 # TODO: use PowerShell to check OS when available
+try {
     $Runtime = [System.Runtime.InteropServices.RuntimeInformation]
     $OSPlatform = [System.Runtime.InteropServices.OSPlatform]
     $Linux = $Runtime::IsOSPlatform($OSPlatform::Linux)
     $OSX = $Runtime::IsOSPlatform($OSPlatform::OSX)
     $Windows = $Runtime::IsOSPlatform($OSPlatform::Windows)
+} catch [System.Management.Automation.RuntimeException] {
+    $Linux = $false
+    $OSX = $false
+    $Windows = $true
+}
 
 function Start-PSBuild
 {