Fix dialyzer errors

[drgmr]

I'm now using this lib as a dependency on a private project and our dialyzer checks complained that a call to ExTwilio.JWT.AccessToken.to_jwt!/1 would make the enclosing function have no local return. After some digging I've found the culprit - the success typing for Joken.generate_and_sign!/3 requires that the second params is a map instead of nil.

That's one of the changes - I've also taken the opportunity to fix the remaining dialyzer errors on the project and added a new step to the Semaphore CI checks so we can ensure no more issues arise.