Avoid dirty submodules for Carthage users
Avoid dirty submodules for Carthage users

Carthage creates the directory `Carthage/Build` for every submodules when using `carthage update --submodule`.
This behavior leads to dirty submodule.
After this patch, `Carthage/Build` in R.swift.Library will be ignored, so the submodule do not get dirty.

mac-cain13/R.swift.Library,uny/R.swift.Library,mac-cain13/R.swift.Library,uny/R.swift.Library