Update powershell.rb

To be clear, the shell that was tested with was 'windows/shell_reverse_tcp' delivered via 'exploit/windows/smb/psexec'

Additional changes required to fix regex to support the multiline output.  Also, InstanceId uses a lower case 'D' on the platforms I tested - PowerShell 2.0 on Windows 2003, Windows 7, Windows 2008 R2 as well as PowerShell 4.0 on Windows 2012 R2.

This method doesn't appear to be used anywhere in the Metasploit codebase currently.