Seeed:Arduino: Add travis build status

README.md

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-Grove_Temperature_And_Humidity_Sensor
+Grove_Temperature_And_Humidity_Sensor  [![Build Status](https://travis-ci.com/Seeed-Studio/Grove_Temperature_And_Humidity_Sensor.svg?branch=master)](https://travis-ci.com/Seeed-Studio/Grove_Temperature_And_Humidity_Sensor)
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 This is an Arduino library for the DHT series of low cost temperature/humidity sensors.