Land rapid7#3390, Fix have_powershell

Author: Meatballs1

lib/msf/core/post/windows/powershell.rb

@@ -19,8 +19,8 @@ module Powershell
   # Returns true if powershell is installed
   #
   def have_powershell?
-    cmd_out = cmd_exec("powershell get-host")
-    return true if cmd_out =~ /Name.*Version.*InstanceID/
+    cmd_out = cmd_exec('cmd.exe /c "echo. | powershell get-host"')
+    return true if cmd_out =~ /Name.*Version.*InstanceId/m
     return false
   end