[MRG] Update README footer

[navinpai]

• Use footer image on Github itself instead of a third party service
• Got the original image from Wayback Archive
• Fix for #380

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@0efb3ca). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #381   +/-   ##
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  Coverage          ?   87.69%           
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  Files             ?       13           
  Lines             ?     1511           
  Branches          ?      349           
=========================================
  Hits              ?     1325           
  Misses            ?      129           
  Partials          ?       57

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[vinayak-mehta]

Thanks for the fix!