Some might say, it's a spec, so don't sweat it. But I think that not to acknowledge these things suggests you don't care, or are stupid and don't know about plagiarism law, or think that it doesn't matter because "it's just a spec".
I say, show that you're a smarta$$ and put in an attribution.
Still, as one of my readers said, of one of my scripts that was full of these (poetry quotes, etc.):
All those references would come out in the wash with a clearance report.
Nothing came of this particular read, but I'm sure he meant, "come out" as in removed, or ensured that the attributions/rights are established correctly by all those expensive studio lawyers.