Update README.md

Add Travis build badge

Author: me-no-dev

README.md

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 # Arduino core for ESP32 WiFi chip
 
+[![Build Status](https://travis-ci.org/espressif/arduino-esp32.svg?branch=master)](https://travis-ci.org/espressif/arduino-esp32)
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 ## Need help or have a question? Join the chat at [![https://gitter.im/espressif/arduino-esp32](https://badges.gitter.im/espressif/arduino-esp32.svg)](https://gitter.im/espressif/arduino-esp32?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 - [Development Status](#development-status)