maint: add travis badge to readme [skip ci]

Author: oesteban

README.md

@@ -3,4 +3,5 @@ A python client to query TemplateFlow via datalad
 
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.2583289.svg)](https://doi.org/10.5281/zenodo.2583289)
 [![CircleCI](https://circleci.com/gh/templateflow/python-client/tree/master.svg?style=shield)](https://circleci.com/gh/templateflow/python-client/tree/master)
+[![Build Status](https://travis-ci.org/templateflow/python-client.svg?branch=master)](https://travis-ci.org/templateflow/python-client)
 [![Pypi](https://img.shields.io/pypi/v/templateflow.svg)](https://pypi.python.org/pypi/templateflow/)