Adding TravisCI build status image

Author: Jeff Ammons

README.md

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 python-rtmbot
 =============
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+[![Build Status](https://travis-ci.org/slackhq/python-rtmbot.png)](https://travis-ci.org/slackhq/python-rtmbot)
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 A Slack bot written in python that connects via the RTM API.
 
 Python-rtmbot is a callback based bot engine. The plugins architecture should be familiar to anyone with knowledge to the [Slack API](https://api.slack.com) and Python. The configuration file format is YAML.