CI: remove Travis configuration (#595)

We've [moved to CircleCI][circ].

[circ]: #594

Author: mockdeep

.travis.yml

@@ -1,6 +1,6 @@
 # Stringer
 
-[![Build Status](https://api.travis-ci.org/swanson/stringer.svg?style=flat)](https://travis-ci.org/swanson/stringer)
+[![CircleCI](https://circleci.com/gh/stringer-rss/stringer/tree/main.svg?style=svg)](https://circleci.com/gh/stringer-rss/stringer/tree/main)
 [![Code Climate](https://codeclimate.com/github/swanson/stringer.svg?style=flat)](https://codeclimate.com/github/swanson/stringer)
 [![Coverage Status](https://coveralls.io/repos/swanson/stringer/badge.svg?style=flat)](https://coveralls.io/r/swanson/stringer)