Autoload Metasploit Payloads Gem

[dwelch-r7]

When defer-module-loads was set to true you would get a warning Could not verify the integrity of the Metasploit Payloads manifest because the Metasploit Payloads gem was not being loaded in time
We're fixing that here by just having the payloads gem be autoloaded

Verification steps

• Boot msfconsole
• Run features set defer_module_loads true
• save, exit and boot console again
• The warning should no longer appear