fix: add Travis-CI badge and trigger new CI builds

Author: oesteban

README.rst

@@ -11,6 +11,9 @@ dmriprep
 .. image:: https://circleci.com/gh/nipreps/dmriprep.svg?style=svg
     :target: https://circleci.com/gh/nipreps/dmriprep
 
+.. image:: https://travis-ci.org/nipreps/dmriprep.svg?branch=master
+    :target: https://travis-ci.org/nipreps/dmriprep
+
 .. image:: https://zenodo.org/badge/DOI/10.5281/zenodo.3392201.svg
     :target: https://doi.org/10.5281/zenodo.3392201