Fix ray-aabb intersection code and test

Before applying this patch one of the tests has been failing:

```
............F....................................
======================================================================
FAIL: test_ray_intersect_aabb (pyrr.test.test_geometric_tests.test_geometric_tests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/aa/projects/Pyrr/pyrr/test/test_geometric_tests.py", line 169, in test_ray_intersect_aabb
    "Ray vs AABB intersection incorrect"
AssertionError: Ray vs AABB intersection incorrect

----------------------------------------------------------------------
Ran 49 tests in 0.150s

FAILED (failures=1)
```