Added Carthage/Build to .gitignore

When using the option `--use-submodules` with Carthage, the `Carthage/Build` directory shows in the submodule after `carthage build` making the submodule dirty.

.gitignore

@@ -23,4 +23,7 @@ DerivedData
 # http://guides.cocoapods.org/using/using-cocoapods.html#should-i-ignore-the-pods-directory-in-source-control
 #
 # Pods/
-.DS_Store
+.DS_Store
+
+# Carthage
+Carthage/Build