ref: #67 only execute post-migration dependent assert for postgresql and django 1.7+
ref: #67 only execute post-migration dependent assert for postgresql and django 1.7+

pretix/cleanerversion,swisscom/cleanerversion,swisscom/cleanerversion,anfema/cleanerversion,pretix/cleanerversion,swisscom/cleanerversion,anfema/cleanerversion,anfema/cleanerversion,pretix/cleanerversion