README.md: Adding Travis CI build banner

Author: Intuity

README.md

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 # Patch Panel
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+[![Build Status](https://travis-ci.org/Intuity/patchpanel.svg?branch=main)](https://travis-ci.org/Intuity/patchpanel)
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 Patch Panel is a Python library for solving constrained connectivity between sources and sinks. The sources and sinks can be any arbitrary (hashable) object, and connectivity is returned as a simple list of tuples. This library does not guarantee a unique solution, it simply returns the first legal solution that satisfies all constraints.
 
 ## Installing