Update resnet18 test value to fix tests

[datumbox]

Change the SUM_OF_PRETRAINED_RESNET18_PARAMS decimal values to resolve broken tests on Travis:

• https://travis-ci.org/github/pytorch/vision/jobs/742436322
• https://travis-ci.org/github/pytorch/vision/jobs/742436323

[codecov]

Codecov Report

Merging #2978 (07d0748) into master (3852b41) will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2978      +/-   ##
==========================================
+ Coverage   73.41%   73.73%   +0.32%     
==========================================
  Files          99       99              
  Lines        8812     8945     +133     
  Branches     1389     1419      +30     
==========================================
+ Hits         6469     6596     +127     
- Misses       1917     1923       +6     
  Partials      426      426              

Impacted Files	Coverage Δ
torchvision/models/detection/mask_rcnn.py	90.76% <0.00%> (+0.14%)	⬆️
torchvision/models/detection/keypoint_rcnn.py	89.04% <0.00%> (+0.15%)	⬆️
torchvision/models/detection/faster_rcnn.py	82.02% <0.00%> (+0.20%)	⬆️
torchvision/models/detection/backbone_utils.py	94.91% <0.00%> (+0.62%)	⬆️
torchvision/ops/deform_conv.py	72.44% <0.00%> (+1.48%)	⬆️
torchvision/models/inception.py	86.85% <0.00%> (+2.21%)	⬆️
torchvision/models/shufflenetv2.py	89.05% <0.00%> (+3.00%)	⬆️
torchvision/ops/feature_pyramid_network.py	94.50% <0.00%> (+3.29%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3852b41...07d0748. Read the comment docs.

[fmassa]

Thanks!