Fix button alignment in portrait mode.

Author: jatsqi

MaterialDialogLibrary/src/main/res/layout/layout_alert_dialog.xml

@@ -53,6 +53,7 @@
                 android:layout_height="wrap_content"
                 android:layout_below="@+id/textView_message"
                 android:layout_marginTop="16dp"
+                android:gravity="center"
                 android:orientation="horizontal"
                 android:paddingLeft="16dp"
                 android:paddingRight="16dp"