change MNIST download links

[CarloLucibello]

Fix #57 using the mirror offered by pytorch vision (pytorch/vision#3544)

@johnnychen94

[johnnychen94]

A good fix to me.

Eventually, I want to switch to Artifacts based to take advantage of Julia's Pkg/Storage server; users in China usually have difficulties connecting to aws s3... but have good connection to Julia Pkg mirrors (e.g., BFSU).

[CarloLucibello]

switching to artifacts would be good, we have to find some hosting service though

[johnnychen94]

we have to find some hosting service though

Github releases :P https://github.com/JuliaML/MLDatasets.jl/releases/tag/v0.6.0-datasets

[codecov-io]

Codecov Report

Merging #58 (176dedc) into master (89fb38b) will decrease coverage by 1.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   11.18%   10.16%   -1.03%     
==========================================
  Files          27       26       -1     
  Lines         626      620       -6     
==========================================
- Hits           70       63       -7     
- Misses        556      557       +1     

Impacted Files	Coverage Δ
src/MNIST/MNIST.jl	66.66% <ø> (-8.34%)	⬇️
src/CIFAR10/CIFAR10.jl	50.00% <0.00%> (-10.00%)	⬇️
src/SVHN2/SVHN2.jl	66.66% <0.00%> (-8.34%)	⬇️
src/CIFAR100/CIFAR100.jl	66.66% <0.00%> (-8.34%)	⬇️
src/FashionMNIST/FashionMNIST.jl	66.66% <0.00%> (-8.34%)	⬇️
src/download.jl	0.00% <0.00%> (-3.23%)	⬇️
src/MNIST/Reader/Reader.jl

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