Version 6.0.0 - Adjust cache keys to be unique per environment

Closes #201. The local storage keys for the SDK cache are now unique for each environment. This is being released as a breaking change because, upon upgrading to this version, the cache will be missed the first time due to the previous key ('BULLET_TRAIN_DB') being ignored

Thank you very much for the great PR @oluizcarvalho 🚀