Fix MySQL error when truncating a table that doesn't exist

This adds a conditional statement before truncating the
`schema_migrations` table.

This comit also ports this code to Rails 5.

lib/tasks/gitlab/db.rake

@@ -29,10 +29,11 @@ namespace :gitlab do
       # If MySQL, turn off foreign key checks
       connection.execute('SET FOREIGN_KEY_CHECKS=0') if Gitlab::Database.mysql?
 
-      tables = connection.tables
+      tables = connection.data_sources
+      # Removes the entry from the array
       tables.delete 'schema_migrations'
       # Truncate schema_migrations to ensure migrations re-run
-      connection.execute('TRUNCATE schema_migrations')
+      connection.execute('TRUNCATE schema_migrations') if connection.data_source_exists? 'schema_migrations'
 
       # Drop tables with cascade to avoid dependent table errors
       # PG: http://www.postgresql.org/docs/current/static/ddl-depend.html