add test dependency ImageIO

[johnnychen94]

This works around the ImageMagick bug in JuliaIO/ImageMagick.jl#206

[codecov]

Codecov Report

Merging #44 (0b09d19) into master (a20cd6b) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage   98.74%   98.77%   +0.03%     
==========================================
  Files           5        5              
  Lines         159      163       +4     
==========================================
+ Hits          157      161       +4     
  Misses          2        2              

Impacted Files	Coverage Δ
src/ssim.jl	100.00% <0.00%> (ø)
src/msssim.jl	100.00% <0.00%> (ø)
src/psnr.jl	96.15% <0.00%> (+0.32%)	⬆️

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