It's an alternate form of the 3pl perf. act. ending; it's pretty common, not just with promulgo, but with all verbs, both in poetry and prose (although it's particularly helpful in poetry, because it helps words to fit the meter).
portāvēre = portāvērunt
censuēre = censuērunt
Dic mihi, Damoeta, 'cuium pecus' anne Latinum?