Double Ratchet Algorithm (Axolotl) implementations

I'm looking for libraries that implement the Double Ratchet Algorithm (known also as Axolotl). So far I've found the following:

• https://github.com/wireapp/proteus
• https://whispersystems.org/docs/

Unfortunately they are both GPL licensed, and I'm looking for something that could be used in Android app without forcing me to make my project open source.

It can be Java or a native C++ library.