Add Homebrew.jl/ImageMagick's bin directory to PATH

This allows issues like https://github.com/timholy/Images.jl/issues/240 to work with `Homebrew.jl`.

Contents of bin directory on OSX 10.10 with ImageMagick 6.8.9:

```
Magick++-config
Magick-config
MagickCore-config
MagickWand-config
Wand-config
animate
compare
composite
conjure
convert
display
identify
import
mogrify
montage
stream
```