Bump jwt to version ~> 2.1

[jbhannah]

2.0.0+ enforce an integer in the payload's exp parameter. One test used an expiration time of 0.01 to simulate an already-expired token, which triggered the raise added in jwt/ruby-jwt#205. A value of -1 accomplishes the same thing and allows the test to pass.

[waiting-for-dev]

Thanks @jbhannah for your collaboration. Do you need a new release with it?