Fix OSX Homebrew build issues and made build script more consistent

[Allardvm]

For a while now, we've had issues with OSX builds breaking, sometimes even resulting in a segfault:

• LoadError: Provider BinDeps.PackageManager failed to satisfy dependency libwand while building ImageMagick JuliaPackaging/Homebrew.jl#175
• Load Issues Cont'd, ImageMagick  JuliaPackaging/Homebrew.jl#176
• ImageMagick problem JuliaPackaging/Homebrew.jl#178
• Pkg.build error on MacOS #66
• Pkg.build("ImageMagick") version 0.2.1 on OS X leads to segmentation fault #74
• LoadError: Provider BinDeps.PackageManager failed to satisfy dependency libwand while building ImageMagick #75

It's likely that these issues have different root causes, but at least one of them has to do with the way Homebrew.jl handles the translation from homebrew/core to staticfloat/juliadeps.

In some cases, the ImageMagick.jl build script would ask for the Homebrew prefix to the homebrew/core/imagemagick@6 library, but the Julia homebrew library would still have the staticfloat/juliadeps version installed. Rebuilding the library would not replace the installed version (despite calling for "homebrew/core/imagemagick@6"), but it would update deps.jl to point to the new version—resulting in errors. After testing and merging of the PR, we should probably tag a new version of the package to fix this for the users that filed issues.

This PR includes three sets of changes:

1. The package always calls for the staticfloat/juliadeps version—both upon the initial installation and upon rebuilding later on. This should prevent inconsistencies between the installed library and deps.jl.
2. The PR also removed the automatic detection of a system-wide Homebrew installed ImageMagick library, since the detect call (success('brew list imagemagick@6') isn't consistent. After loading the Homebrew package in Julia, the detect call would call the Julia Homebrew, rather than the system-wide Homebrew—which in rare cases would lead to further inconsistencies. The PR now uses the MAGICK_HOME environment variable to allow users to specify their own installation of ImageMagick—similar to the Unix version of the package.
3. General formatting and consistency fixes. Each platform now uses a similar build script to make maintaining the package a bit easier.

Note that some users that have tried to fix the issue manually in the past by taking control of Julia's Homebrew might have to manually remove the library installed by Homebrew.jl, prune the remaining symlinks, and rebuild the ImageMagick package:

using Homebrew
run(`brew remove imagemagick@6`)
run(`brew prune`)
Pkg.build("ImageMagick")

[Allardvm]

The failing nightly builds are due to an issue in ZipFile.jl that seems to have been fixed in fhs/ZipFile.jl@b90db13. A release with this fix hasn't been tagged yet. Let me know how you'd like me to handle that.

[timholy]

Awesome. First thing: can you add 0.6 to the testing matrices? You surely know this already, but here and here (two lines worth in the latter case). On 0.6, ZipFile shouldn't fail because the alignment issue hasn't been merged. Assuming 0.6 passes, I'm fine with nightly failing until ZipFile gets tagged.

[timholy]

Puzzling. Exit code 56 is "Failure with receiving network data," https://curl.haxx.se/libcurl/c/libcurl-errors.html. I'm going to restart in case it was just a temporary outage.

[Allardvm]

This looks unrelated to the changes, perhaps it's a DNS issue again, somewhere? The OSX 0.5 version works this time, perhaps you could restart OSX 0.6 as well to make sure it passes?

[timholy]

This is one of those PRs where there aren't enough happy emojis to go around. Many thanks, @Allardvm!

[Allardvm]

You're welcome! Thanks for the work to get this merged so quickly 👍