gitignore: ignore Carthage build folder

This patch updates the .gitignore file to ignore the folder where
Carthage builds the frameworks.

Change-Id: I0273e312469acaeadb440459c8560d1ec8f92fc6
Tuleap: #1377

.gitignore

@@ -21,3 +21,6 @@ Ring/Ring.xcodeproj/xcuserdata/
 Pods/
 
 Ring/Build/
+
+# Carthage
+Ring/Carthage/