Update devise-jwt.gemspec to make it compatible with devise 5 alpha

[chaadow]

Summary

Devise started the development of devise 5 in its alpha phase. I believe it's safe to have a less strict on the dependency's version

This is also so that people using devise's main branch can still use devise-jwt

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[waiting-for-dev]

Hey @chaadow, I think that would be risky and the appropriate thing to do would be maintaining a branch with support for devise 5 in the meantime. What do you think?

[chaadow]

merged inside #284 will be automatically closed/merged once #284 is merged