Delete forked expo-server-sdk-ruby gem dependency

[sanak]

Problem
Currently, gem includes the following forked repository, but this needs additional git package installation when building time.
https://github.com/gtt-project/expo-server-sdk-ruby

Improvement
Switch to use upstream expo-server-sdk-ruby official gem, because critical issue seems to be solved at last November (expo-community/expo-server-sdk-ruby#36 (comment)).