update fixture test/expected/a.0.0.0.png

Closes #15 

Not seeing any visible differences here other than the file size doubling, am I missing a dependency?

In `graphicsmagick` branch:
```
1) tiles format a (0.0.0) as png:
   Error: Image size is too different from fixture: 22846 vs. 11541
```

Running `mapnik 2.2.0`, `libpng 1.5.17` and `freetype 2.5.1` (per https://github.com/mapbox/tilestream-pro/issues/3290#issuecomment-29855338)

/cc @willwhite @springmeyer