Update voprf dependency in Cargo.toml to point to a published version

[kevinlewi]

Will create an issue to make sure we don't forget to update this

Originally posted by @kevinlewi in #255 (comment)

[daxpedda]

I'm pretty much done with improving/fixing voprf. facebook/voprf#55, facebook/voprf#56 and facebook/voprf#54 are the last remaining PR's and then we can cut a release.