spacesuit: fix the trait bound for the RngCore

[oleganza]

This fixes the issue triggered by the recent curve25519-dalek update to v1.1.

This is the issue:

error[E0382]: use of moved value: `rng`
   --> src/value.rs:161:70
    |
160 |         let (q_commit, q_var) = prover.commit(self.q.into(), Scalar::random(rng));
    |                                                                             --- value moved here
161 |         let (f_commit, f_var) = prover.commit(self.f, Scalar::random(rng));
    |                                                                      ^^^ value used here after move
    |
    = note: move occurs because `rng` has type `&mut R`, which does not implement the `Copy` trait

An issue on curve25519-dalek: dalek-cryptography/curve25519-dalek#232

[vickiniu]

LGTM!

[oleganza]

Henry published a v1.1.3 that reverts the API change that broke spacesuit. We should review the rng-using APIs in a more principled manner. For now just do cargo update to pick up curve25519-dalek v1.1.3.