Revert 14514d7 as well

Another one snuck in there, see #4175

Revert "update gemspec for metasploit-credential"

This reverts commit 14514d7b8b65489066b978311e7be40b61ec543d, reversing
changes made to 6653d5e9139607cf9489482c5e7f30e07fdd2bc8.
Land #4176, revert of 14514d7