Update README.md

rm coverall

Author: Roger-luo

README.md

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 [![Build Status](https://travis-ci.com/QuantumBFS/YaoBlocks.jl.svg?branch=master)](https://travis-ci.com/QuantumBFS/YaoBlocks.jl)
 [![Codecov](https://codecov.io/gh/QuantumBFS/YaoBlocks.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/QuantumBFS/YaoBlocks.jl)
-[![Coveralls](https://coveralls.io/repos/github/QuantumBFS/YaoBlocks.jl/badge.svg?branch=master)](https://coveralls.io/github/QuantumBFS/YaoBlocks.jl?branch=master)
 
 Standard basic quantum circuit simulator building blocks.