libsodium 0.4.3 support

It appears there was a minor change in the ABI:

crypto_onetimeauth_poly1305_ref(_*) => crypto_onetimeauth_poly1305(_*)
Restore "Update Rake tasks to libsodium 0.4.3"

This reverts commit ad1cd177d52e897161c547964e8a1e0374271128.