Run migrations using php artisan

[duncanmcclean]

This pull request fixes an error when creating new sites w/ a database:

 Which database will your application use? ───────────────────┐
 │ MySQL                                                        │
 └──────────────────────────────────────────────────────────────┘


                                               
  Not enough arguments (missing: "timezone").  
                                               

   There was a problem connecting to the database. 

  Once the install process is complete, please run php please install:eloquent-driver to finish setting up the database.

This was happening because...

1. We were running php please migrate.
2. The statamic:migrate-dates-to-utc command becomes migrate-dates-to-utc in Please.
3. Symfony's console uses abbreviation matching, meaning migrate is a valid abbreviation/prefix of migrate-dates-to-utc.
4. It meant that php please migrate would end up matching migrate-dates-to-utc, rather than falling back to Laravel's migrate command, like it did < v6.
5. The command requires a timezone argument, hence the error.

This PR fixes it by explicitly using php artisan to run the migration command, rather than rely on the fallback behaviour of php please.

Fixes #101