Fix tests and git migration gathering

django_migration_linter/migration.py

@@ -5,3 +5,8 @@ class Migration(object):
     def __init__(self, abs_path):
         self.abs_path = abs_path
         self.app_name, self.name = split_migration_path(self.abs_path)
+
+    def __lt__(self, other):
+        if not isinstance(other, Migration):
+            return NotImplemented
+        return self.abs_path < other.abs_path