add travis badge

Author: machow

README.md

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 ![banner](https://s3.amazonaws.com/assets.datacamp.com/img/github/content-engineering-repos/pythonwhat_banner_v2.png)
 
+[![Build Status](https://travis-ci.org/datacamp/pythonwhat.svg?branch=master)](https://travis-ci.org/datacamp/pythonwhat)
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 The `pythonwhat` package provides rich functionality to write Submission Correctness Tests for interactive Python exercises on the DataCamp platform. DataCamp operates with **Python 3**.
 
 For a detailed guide on how to use `pythonwhat`, head over to [the online documentation](http://pythonwhat.readthedocs.io). Before, all documentation was on the [wiki](https://github.com/datacamp/pythonwhat/wiki), but things are steadily being moved to the _readthedocs_ format.