Re-fixes issue #12, reintroduced due to 286138d

[oschulz]

Commit 286138d removed the fix for issue #12. Observed on CentOS-7 with system-installed ImageMagick.

[timholy]

Thanks. Hadn't realized this might still be an issue.

[Allardvm]

I'm not a huge fan of the somewhat hacky fix, because we don't fix the root cause. I assume the error is thrown from __init__(), because the preload and unload hooks aren't added to deps.jl when the library is found in a system path. If this is the case, then wouldn't the below also fix the issue?

1. remove the call to init_deps from __init__().
2. add ccall((:MagickWandGenesis, libwand), Void, ()) to __init__().
3. remove the ccall from all the init_deps() functions defined by the BinDeps providers.
4. move all the init_deps() functions to each provider's preload function, rather than the onload function to ensure the ENV setup is in place when the library is first opened.
   update: no, this doesn't work because the package is precompiled

If you're on-board with this, I'm happy to open a PR that does this. I'll need to open a PR anyway, since #90 didn't turn out to be a final fix for the Homebrew library issue. Even though the package now always calls for Homebrew's staticfloat/juliadeps/imagemagick@6 library, it just installed homebrew/core/imagemagick@6 for me again after calling Pkg.update(). So I'll have to try a different approach.

[oschulz]

@timholy, will v0.3.1 go on METADATA? Would help me and some folks I work with a lot.