add travis status flag

README.md

 
 # CATH-SM Server
 
+[![Build Status](https://travis-ci.com/CATH-SWISSMODEL/cath-swissmodel-api.svg?branch=master)](https://travis-ci.com/CATH-SWISSMODEL/cath-server)
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 This repository contains scripts and libraries for running the backend Django server
 required for the CATH-SM (CATH/SWISS-MODEL) protein sequence modelling pipeline.