JWT exp value is set to "now + 1 hour" unlike all the other Universal SDKs #65

Description

The JWTs created by this SDK have their exp value set to "now +1 hour", but all the other Universal SDK libraries use "now +5 minutes". Duo has recently begun enforcing that exp values more than 1 hour in the future will be rejected. which means if the server clock is even 1 second ahead of Duo's, auths will fail.

Expected Behavior

JWT exp times should be set to "now +5 minutes" to be consistent with the other SDKs.

Actual Behavior

JWT exp times is "now +1 hour".

Steps to Reproduce

1. Have a server time that is a minute or two fast
2. Initiate an auth to Duo
3. Observe the auth is rejected

Workarounds

Set the server time to be slow by about a minute.