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.

Author: Kuniwak

.gitignore

@@ -6,3 +6,4 @@ fastlane/report.xml
 fastlane/test_output
 fastlane/settoken.sh
 /build
+Carthage/Build