References
왕초보를 위한 “Private” CocoaPods Library만들기
Cocoapods Private 라이브러리에 관한 튜토리얼
https://guides.cocoapods.org/making/private-cocoapods.html
https://guides.cocoapods.org/syntax/podspec.html#subspec
https://medium.com/@euginedubinin/development-pods-de631774751e
Error handling
$ sudo rm -fr ~/.cocoapods/repos/master
$ pod setup