ImageMagick overrides existing executables in PATH

[anthony-wang]

Platform: Win10, Julia v0.6.2, ImageMagick v0.5.1

When using ImageMagick, it overrides existing bindings in PATH and does it silently. This can cause unexpected behaviour when the binary from ImageMagick and the originally existing binary from PATH are not the same.

julia> run(`where ffmpeg`)
C:\ProgramData\chocolatey\bin\ffmpeg.exe

julia> run(`ffmpeg -version`)
ffmpeg version 4.0 Copyright (c) 2000-2018 the FFmpeg developers

julia> using ImageMagick

julia> run(`where ffmpeg`)
C:\Users\Anthony\.julia\v0.6\ImageMagick\deps\usr\lib\x64\{app}\ffmpeg.exe
C:\ProgramData\chocolatey\bin\ffmpeg.exe

julia> run(`ffmpeg -version`)
ffmpeg version 3.4 Copyright (c) 2000-2017 the FFmpeg developers

Thanks to @SimonDanisch for helping debug this!

[SimonDanisch]

Should be fixed by #121!