Fix reader 0007 migration not working in MySQL

The migration was generating this SQL:

    ALTER TABLE `reader_series` MODIFY `id`
    integer AUTO_INCREMENT NOT NULL;
    ALTER TABLE `reader_series` ADD CONSTRAINT
    `reader_series_id_1242ab74_uniq` UNIQUE (`id`);
    ALTER TABLE `reader_series` ADD CONSTRAINT
    `reader_series_id_1242ab74_pk` PRIMARY KEY (`id`);

but MySQL requires auto fields to be declared as primary keys.

reader/migrations/0007_editable_slugs.py

@@ -11,10 +11,6 @@ def __init__(self, name, app_label):
             ('reader', '0006_remove_chapter_url'),
         ]
         self.operations = [
-            migrations.AddField(
-                model_name='series', name='id',
-                field=models.IntegerField(default=0),
-            ),
             migrations.AlterField(
                 model_name='chapter', name='series',
                 field=models.CharField(max_length=250, blank=True)
@@ -27,6 +23,19 @@ def __init__(self, name, app_label):
             migrations.RemoveField(model_name='series', name='authors'),
             migrations.RemoveField(model_name='series', name='artists'),
             migrations.RemoveField(model_name='series', name='categories'),
+            migrations.AlterField(
+                model_name='series', name='slug',
+                field=models.SlugField(
+                    primary_key=False, blank=True,
+                    unique=True, verbose_name='Custom slug',
+                    help_text='The unique slug of the series.'
+                              ' Will be used in the URL.'
+                )
+            ),
+            migrations.AddField(
+                model_name='series', name='id',
+                field=models.AutoField(primary_key=True, serialize=False)
+            ),
             migrations.RunPython(self.populate_ids),
             migrations.AddField(
                 model_name='series', name='artists',
@@ -40,19 +49,6 @@ def __init__(self, name, app_label):
                 model_name='series', name='categories',
                 field=models.ManyToManyField(blank=True, to='reader.Category'),
             ),
-            migrations.AlterField(
-                model_name='series', name='id',
-                field=models.AutoField(primary_key=True, serialize=False),
-            ),
-            migrations.AlterField(
-                model_name='series',
-                name='slug',
-                field=models.SlugField(
-                    blank=True, unique=True, verbose_name='Custom slug',
-                    help_text='The unique slug of the series.'
-                    ' Will be used in the URL.'
-                )
-            ),
             migrations.RunPython(self.populate_m2m_fields),
             migrations.AlterField(
                 model_name='chapter', name='series',
@@ -85,10 +81,7 @@ def populate_ids(self, apps, schema_editor):
         series = apps.get_model('reader', 'Series')
         chapter = apps.get_model('reader', 'Chapter')
         alias = apps.get_model('reader', 'SeriesAlias')
-        pk = 1
         for s in series.objects.all():
-            s.id = pk
-            pk += 1
             s.save()
             for c in chapter.objects.filter(series=s.slug):
                 c.series = s.id