Fix imports

src/main/java/com/owncloud/android/ui/fragment/contactsbackup/ContactsBackupFragment.java

@@ -31,6 +31,7 @@ import android.os.AsyncTask;
 import android.os.Bundle;
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
+import android.support.design.widget.Snackbar;
 import android.support.v4.app.Fragment;
 import android.support.v7.app.ActionBar;
 import android.support.v7.widget.AppCompatButton;