fix rake stats

```bash

rake stats
+----------------------+-------+-------+---------+---------+-----+-------+
| Name                 | Lines | LOC   | Classes | Methods | M/C | LOC/M |
+----------------------+-------+-------+---------+---------+-----+-------+
| Prawn                |  9164 |  4725 |      44 |     583 |  13 |     6 |
| Specs                |  8784 |  7317 |       6 |     741 | 123 |     7 |
| spec/acceptance      |    25 |    17 |       0 |       0 |   0 |     0 |
| spec/extensions      |    57 |    29 |       2 |       6 |   3 |     2 |
+----------------------+-------+-------+---------+---------+-----+-------+
| Total                | 18030 | 12088 |      52 |    1330 |  25 |     7 |
+----------------------+-------+-------+---------+---------+-----+-------+
 Code LOC: 4725  Test LOC: 7363  Code to Test Ratio: 1:1.6

```