Improve database seeders to avoid conflicts

[quintonpryce]

The error that was run into:

vagrant@homestead:~/Code/core$ php artisan db:seed
Seeding: UsersTableSeeder
Seeding: CoursesTableSeeder

                                                                               
  [Illuminate\Database\QueryException]                                         
  SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'repo  
  sitories/MATH8756/A2/76/' for key 'repositories_path_unique' (SQL: insert i  
  nto `repositories` (`name`, `path`, `backend`, `updated_at`, `created_at`)   
  values (A2, repositories/MATH8756/A2/76/, git, 2017-03-30 17:45:44, 2017-03  
  -30 17:45:44))                                                               
                                                                               

                                                                               
  [PDOException]                                                               
  SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'repo  
  sitories/MATH8756/A2/76/' for key 'repositories_path_unique'                 
                                                                               

vagrant@homestead:~/Code/core$ php artisan migrate:rollback
Rolling back: 2017_03_15_004119_create_submissions_table
Rolled back: 2017_03_15_004119_create_submissions_table
Rolling back: 2017_03_15_002742_create_assignments_table
Rolled back: 2017_03_15_002742_create_assignments_table
Rolling back: 2017_03_09_015134_create_repositories_table
Rolled back: 2017_03_09_015134_create_repositories_table
Rolling back: 2017_02_15_034244_create_courses_users_table
Rolled back: 2017_02_15_034244_create_courses_users_table
Rolling back: 2017_02_14_204134_create_courses_table
Rolled back: 2017_02_14_204134_create_courses_table
Rolling back: 2014_10_12_100000_create_password_resets_table
Rolled back: 2014_10_12_100000_create_password_resets_table
Rolling back: 2014_10_12_000000_create_users_table
Rolled back: 2014_10_12_000000_create_users_table
vagrant@homestead:~/Code/core$ php artisan migrate
Migrating: 2014_10_12_000000_create_users_table
Migrated: 2014_10_12_000000_create_users_table
Migrating: 2014_10_12_100000_create_password_resets_table
Migrated: 2014_10_12_100000_create_password_resets_table
Migrating: 2017_02_14_204134_create_courses_table
Migrated: 2017_02_14_204134_create_courses_table
Migrating: 2017_02_15_034244_create_courses_users_table
Migrated: 2017_02_15_034244_create_courses_users_table
Migrating: 2017_03_09_015134_create_repositories_table
Migrated: 2017_03_09_015134_create_repositories_table
Migrating: 2017_03_15_002742_create_assignments_table
Migrated: 2017_03_15_002742_create_assignments_table
Migrating: 2017_03_15_004119_create_submissions_table
Migrated: 2017_03_15_004119_create_submissions_table
vagrant@homestead:~/Code/core$ php artisan db:seed
Seeding: UsersTableSeeder
Seeding: CoursesTableSeeder
Seeding: RepositoryTableSeeder
vagrant@homestead:~/Code/core$