Lewis' definition of "convention" (p. 78) crucially relies on the concept of common knowledge as well as rationality: For a equilibrium R to be a convention, it must be common knowledge to everyone that the majority conforms to R.
So mere behavioral adaption without a mutual ascription of rationality doesn't count as "convention" (cf. p. 59). Every player has to think that every other player is rational and has the same knowledge as him- or herself. You can't think that you're the only one who's not a robot and still call it a "convention" according to Lewis.
He also excludes solutions to trivial coordination games (p. 78, item 5). There has to be at least two distinct equilibria available so that the fixation in one or the other becomes truly contingent. He doesn't seem to entertain the possibility that there could be something like degrees of contingency.
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