Added npm badge

README.md

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 [![Build Status](https://api.travis-ci.com/indexed-finance/uniswap-v2-oracle.svg?branch=master)](https://travis-ci.com/github/indexed-finance/uniswap-v2-oracle)
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 While this contract was developed for use in the Indexed protocol, the name comes from the fact that it indexes keys to track timestamps that the oracle has observed prices for.