Fix 'No Rakefile found' for rails:{db_create, db_migrate, db_rollback, assetts_precompile} commands

[ianheggie]

I like the gem, but there are rails:* commands that fail when run alone, eg:

  $ mina rails:db_create
  -----> Creating database
         rake aborted!
         No Rakefile found (looking for: rakefile, Rakefile, rakefile.rb, Rakefile.rb)
   
         Connection to licapture.timelapse.com.au closed.

         (See full trace by running task with --trace)
   !     Run Error

I will submit a PR

[d4be4st]

Answer in #525 (comment)

Will add to documentation.
If you feel there is a need to create separate mina tasks for this, go ahead.