lalrpop-snap: Remove dependency on docopt, rustc-serialize.
lalrpop-snap: Remove dependency on docopt, rustc-serialize.

These weren't used here and the dependencies are about to get
updated, so just remove them from here instead.

tempbottle/lalrpop,nikomatsakis/lalrpop,nikomatsakis/lalrpop,tempbottle/lalrpop