Build failure with latest version due to ring

[kpcyrd]

The error I get:

error: multiple packages link to native library `ring-asm`, but a native library can be linked only once

package `ring v0.12.1`
    ... which is depended on by `lamport_sigs v0.5.0`
    ... which is depended on by `merkle_sigs v1.5.0`
    ... which is depended on by `rusty_secrets v0.2.2 (file:///xxx)`
    ... which is depended on by `foo v0.1.0 (file:///xxx)`
links to native library `ring-asm`

package `ring v0.13.2`
    ... which is depended on by `merkle v1.10.0`
    ... which is depended on by `merkle_sigs v1.5.0`
    ... which is depended on by `rusty_secrets v0.2.2 (file:///xxx)`
    ... which is depended on by `foo v0.1.0 (file:///xxx)`
also links to native library `ring-asm`

I've submitted a PR to update lamport_sigs:
SpinResearch/lamport_sigs.rs#15

I've tried to do the same for merkle_sigs, but the current master depends on a different version of merkle.rs that I couldn't build on my system due to a build failure in a dependency:
SpinResearch/merkle.rs#47