switch to GetVirtualPixels

AcquireImagePixels is deprecaed http://www.imagemagick.org/api/deprecate.php#AcquireImagePixels

This fixes the deprecations warnings.  As discussed in #13, this does not make magick thread-safe.  From what I understand, the Go scheduler only allows one goroutine to call across the cgo bridge.  So refactoring the entire codebase to make it thread-safe is premature right now.

```
./color.c:36:13: warning: 'AcquireImagePixels' is deprecated [-Wdeprecated-declarations]
./macros.h:9:50: note: expanded from macro 'ACQUIRE_IMAGE_PIXELS'
/usr/local/Cellar/imagemagick/6.8.9-5/include/ImageMagick-6/magick/deprecate.h:120:4: note: 'AcquireImagePixels' has been explicitly marked deprecated here
# github.com/drewwells/magick
./compat.c:18:13: warning: 'AcquireImagePixels' is deprecated [-Wdeprecated-declarations]
./macros.h:9:50: note: expanded from macro 'ACQUIRE_IMAGE_PIXELS'
/usr/local/Cellar/imagemagick/6.8.9-5/include/ImageMagick-6/magick/deprecate.h:120:4: note: 'AcquireImagePixels' has been explicitly marked deprecated here
# github.com/drewwells/magick
./gif.c:230:13: warning: 'AcquireImagePixels' is deprecated [-Wdeprecated-declarations]
./macros.h:9:50: note: expanded from macro 'ACQUIRE_IMAGE_PIXELS'
/usr/local/Cellar/imagemagick/6.8.9-5/include/ImageMagick-6/magick/deprecate.h:120:4: note: 'AcquireImagePixels' has been explicitly marked deprecated here
./gif.c:251:37: warning: 'AcquireImagePixels' is deprecated [-Wdeprecated-declarations]
./macros.h:9:50: note: expanded from macro 'ACQUIRE_IMAGE_PIXELS'
/usr/local/Cellar/imagemagick/6.8.9-5/include/ImageMagick-6/magick/deprecate.h:120:4: note: 'AcquireImagePixels' has been explicitly marked deprecated here
# github.com/drewwells/magick
./matrix.c:13:13: warning: 'AcquireImagePixels' is deprecated [-Wdeprecated-declarations]
./macros.h:9:50: note: expanded from macro 'ACQUIRE_IMAGE_PIXELS'
/usr/local/Cellar/imagemagick/6.8.9-5/include/ImageMagick-6/magick/deprecate.h:120:4: note: 'AcquireImagePixels' has been explicitly marked deprecated here
```