Add npm version badge to readme

Author: flexdinesh

README.md

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 # Axios Retry Interceptor
 [![Build Status](https://travis-ci.org/flexdinesh/axios-retry-interceptor.svg?branch=master)](https://travis-ci.org/flexdinesh/axios-retry-interceptor)
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 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 
 Configurable Axios Interceptor to retry failed http calls.