Exclude south_migrations from test coverage

.coveragerc

@@ -5,6 +5,7 @@ branch = True
 source = wagtail
 
 omit =
+    */south_migrations/*
     */migrations/*
     wagtail/vendor/*
 
@@ -31,4 +32,4 @@ exclude_lines =
 ignore_errors = True
 
 [html]
-directory = coverage_html_report
+directory = coverage_html_report