📚 docs(README): Use coveralls badge instead of codecov.

Author: make-github-pseudonymous-again

Diff for: README.md

@@ -24,7 +24,7 @@ a.mul( b ).toString( ) ; // '-903954021577363596419770144565091'
 
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+[![Code coverage (alls)](https://img.shields.io/coveralls/github/aureooms/js-integer/master.svg)](https://coveralls.io/r/aureooms/js-integer)
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