Error while decoding the JWT token with signature

[Gokul595]

There is an issue on decoding the JWT token with signature while initializing the MarketingCloudSDK::Client where the decode method returns an array [payload, header] but the code in the gem is accessing as hash which raises the error TypeError (no implicit conversion of String into Integer): after line number 87.

The JWT gem version in jwt 1.5.4

I have fixed the issue and created a pull request #63 with the fix.