Add Travis badge

Author: ccascone

README.md

@@ -147,3 +147,5 @@ Click on the exercise name to see the instructions:
 
 You can find solutions for each exercise in the [solution](solution) directory.
 Feel free to compare your solution to the reference one whenever you feel stuck.
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+[![Build Status](https://travis-ci.org/opennetworkinglab/ngsdn-tutorial.svg?branch=master)](https://travis-ci.org/opennetworkinglab/ngsdn-tutorial)