Sort feature lists for fingerprint hashing

Otherwise the order was nondeterministic likely due to some hash map along the
way being used to deduplicate the set of features.

Closes #665
auto merge of #673 : alexcrichton/cargo/issue-665, r=brson

Otherwise the order was nondeterministic likely due to some hash map along the
way being used to deduplicate the set of features.

Closes #665