disable download tests in Travis CI

[pmeier]

As requested by @fmassa in #2665 (comment).

[codecov]

Codecov Report

Merging #2674 into master will increase coverage by 1.85%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2674      +/-   ##
==========================================
+ Coverage   70.56%   72.41%   +1.85%     
==========================================
  Files          95       95              
  Lines        8245     8245              
  Branches     1308     1308              
==========================================
+ Hits         5818     5971     +153     
+ Misses       2003     1858     -145     
+ Partials      424      416       -8     

Impacted Files	Coverage Δ
torchvision/io/video.py	69.82% <0.00%> (+1.18%)	⬆️
torchvision/models/detection/rpn.py	93.13% <0.00%> (+2.57%)	⬆️
torchvision/models/detection/roi_heads.py	82.34% <0.00%> (+5.10%)	⬆️
torchvision/__init__.py	78.12% <0.00%> (+6.25%)	⬆️
torchvision/datasets/video_utils.py	84.84% <0.00%> (+7.27%)	⬆️
torchvision/ops/poolers.py	97.05% <0.00%> (+11.76%)	⬆️
torchvision/ops/boxes.py	100.00% <0.00%> (+15.55%)	⬆️
torchvision/models/detection/transform.py	95.55% <0.00%> (+17.22%)	⬆️
torchvision/io/_video_opt.py	47.50% <0.00%> (+26.25%)	⬆️
torchvision/ops/_register_onnx_ops.py	85.36% <0.00%> (+36.58%)	⬆️

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[fmassa]

Thanks!