[TASK] update sjcl

- native support for k256
- improved entropy by taking advantage of platform crypto

- remove unnecessary sjcl overrides from sjcl-secp256k1.js
- updated ripple-lib curve instantiations to use k256
- add curve override so c256 points to k256

https://github.com/bitwiseshiftleft/sjcl/commit/16dde36fa2a58845f639429acd5bc4e4ebb5a0f1
Merge pull request #220 from geertweening/update_sjcl

[TASK] update sjcl