Fix require on Python bind_tcp stager

alphis at Freenode #corelan has spot the next issue (http://pastebin.com/SgDjni05) when loading the Python bind_tcp stager on Linux environments:

```
/home/user/metasploit-framework/modules/payloads/stagers/python/bind_tcp.rb:25:in `initialize': uninitialized constant Msf::Handler::BindTcp (NameError)
        from /home/user/metasploit-framework/lib/msf/core/payload_set.rb:198:in `new'
        from /home/user/metasploit-framework/lib/msf/core/payload_set.rb:198:in `add_module'
        from /home/user/metasploit-framework/lib/msf/core/module_manager/loading.rb:72:in `on_module_load'
        from /home/user/metasploit-framework/lib/msf/core/modules/loader/base.rb:207:in `load_module'
        from /home/user/metasploit-framework/lib/msf/core/modules/loader/base.rb:271:in `block in load_modules'
        from /home/user/metasploit-framework/lib/msf/core/modules/loader/directory.rb:58:in `block (2 levels) in each_module_reference_name'
        from /home/user/metasploit-framework/lib/rex/file.rb:127:in `block in find'
        from /home/user/metasploit-framework/lib/rex/file.rb:126:in `catch'
        from /home/user/metasploit-framework/lib/rex/file.rb:126:in `find'
        from /home/user/metasploit-framework/lib/msf/core/modules/loader/directory.rb:45:in `block in each_module_reference_name'
        from /home/user/metasploit-framework/lib/msf/core/modules/loader/directory.rb:29:in `foreach'
        from /home/user/metasploit-framework/lib/msf/core/modules/loader/directory.rb:29:in `each_module_reference_name'
        from /home/user/metasploit-framework/lib/msf/core/modules/loader/base.rb:264:in `load_modules'
        from /home/user/metasploit-framework/lib/msf/core/module_manager/loading.rb:118:in `block in load_modules'
        from /home/user/metasploit-framework/lib/msf/core/module_manager/loading.rb:116:in `each'
        from /home/user/metasploit-framework/lib/msf/core/module_manager/loading.rb:116:in `load_modules'
        from /home/user/metasploit-framework/lib/msf/core/module_manager/module_paths.rb:56:in `block in add_module_path'
        from /home/user/metasploit-framework/lib/msf/core/module_manager/module_paths.rb:55:in `each'
        from /home/user/metasploit-framework/lib/msf/core/module_manager/module_paths.rb:55:in `add_module_path'
        from /home/user/metasploit-framework/lib/msf/base/simple/framework/module_paths.rb:14:in `init_module_paths'
        from /home/user/metasploit-framework/lib/msf/ui/console/driver.rb:228:in `initialize'
        from ./msfconsole:169:in `new'
        from ./msfconsole:169:in `<main>'
```

This patch should fix (tested with alphis, thanks!)