Missing require forwardable

The file is extending the forwardable but forwarable is not require(d) which results in stack trace:

```bash
/opt/metasploit-framework/lib/rex/exploitation/powershell/script.rb:16:in `<class:Script>': uninitialized constant Rex::Exploitation::Powershell::Script::Forwardable (NameError)
        from /opt/metasploit-framework/lib/rex/exploitation/powershell/script.rb:8:in `<module:Powershell>'
        from /opt/metasploit-framework/lib/rex/exploitation/powershell/script.rb:7:in `<module:Exploitation>'
        from /opt/metasploit-framework/lib/rex/exploitation/powershell/script.rb:6:in `<module:Rex>'
        from /opt/metasploit-framework/lib/rex/exploitation/powershell/script.rb:5:in `<top (required)>'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /opt/metasploit-framework/lib/rex/exploitation/powershell.rb:8:in `<top (required)>'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /opt/metasploit-framework/lib/rex/text.rb:6:in `<top (required)>'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /opt/metasploit-framework/lib/rex.rb:44:in `<top (required)>'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from ./msfcli:15:in `<main>'
```