add Travis build badge to the README file

README.rst

+edx-proctoring [![Build Status](https://travis-ci.org/edx/edx-proctoring.svg?branch=master)](https://travis-ci.org/edx/edx-proctoring) [![Coverage Status](https://img.shields.io/coveralls/edx/edx-proctoring.svg)](https://coveralls.io/r/edx/edx-proctoring?branch=master)
+
+========================
+
 This is the Exam Proctoring subsytem for the Open edX platform.