Add a build status badge to the README
This includes a pretty icon badge to the README showing the current
build status for bandit on brach master from Travis CI.
Bandit
======
+.. image:: https://travis-ci.org/PyCQA/bandit.svg?branch=master
+ :target: https://travis-ci.org/PyCQA/bandit/
+ :alt: Build Status
+
.. image:: https://img.shields.io/pypi/v/bandit.svg
:target: https://pypi.org/project/bandit/
:alt: Latest Version
