Bump `parity-crypto` from v0.4 to v0.6

Fixes error when trying to compile tests. This was caused by
`parity-crypto` v0.4's use of `parity-secp256k1` over `secp256k1'.
Using the Parity fork meant multiple version of the same underlying
C library were being pulled in. `parity-crypto` v0.6 moved away from
this, only relying on `secp256k1` thus fixing the issue.