Update README.md

Author: mauriciogtec

README.md

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 [![Build Status](https://travis-ci.org/mauriciogtec/AdaptiveRejectionSampling.jl.svg?branch=master)](https://travis-ci.org/mauriciogtec/AdaptiveRejectionSampling.jl)
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+[![MIT Licence](https://badges.frapsoft.com/os/mit/mit.png?v=103)](https://opensource.org/licenses/mit-license.php)
 [![Coverage Status](https://coveralls.io/repos/github/mauriciogtec/AdaptiveRejectionSampling.jl/badge.svg?branch=master)](https://coveralls.io/github/mauriciogtec/AdaptiveRejectionSampling.jl?branch=master)
 [![codecov](https://codecov.io/gh/mauriciogtec/AdaptiveRejectionSampling.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/mauriciogtec/AdaptiveRejectionSampling.jl)