Support stable rust

Updated dependencies such that Rust stable 1.53 is now supported.
The optimised avx_2 option will NOT rust on stable because there's
still an unstable feature on subtle-ng. BUT this feature is actually
for doc generation and has been removed from Rust. As soon as subtle-ng
merges dalek-cryptography/subtle#85, avx2 will
probably be supported on stable as well.

The rand dep update breaks the benchmarks, but these will be fixed in
the next commit.

Author: CjS77

src/ristretto/dalek_range_proof.rs

@@ -42,6 +42,7 @@ use bulletproofs::{
     BulletproofGens,
     PedersenGens,
     RangeProof as DalekProof,
+
 };
 use merlin::Transcript;