Extract option require pattern to helper Module

MSP-10905

`Metasplot::Framework::Require.optionally` can be used to optionally
require a library and then issue a warning if the require fails or run a
block when it succeeds.
Optionally require 'active_record/railtie` in Rakefile

MSP-10905

Require in Rakefile so that db tasks are defined.  Make it optionally so
that `rake` doesn't error out when metasploit-framework installed with
`bundle install --without db`.