remove Travis CI status badge from README (#3230)

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Co-authored-by: Francisco Massa <fvsmassa@gmail.com>

README.rst

 torchvision
 ===========
 
-.. image:: https://travis-ci.org/pytorch/vision.svg?branch=master
-    :target: https://travis-ci.org/pytorch/vision
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 .. image:: https://codecov.io/gh/pytorch/vision/branch/master/graph/badge.svg
     :target: https://codecov.io/gh/pytorch/vision