Remove Travis CI configuration and badge (#5, thanks @striezel)

The tests are not running on Travis CI anymore, so there is no
point in keeping that stuff around.

[skip ci]

Author: striezel

.travis.yml

@@ -1,6 +1,6 @@
 # boolean lite: A strong boolean type for C++98 and later
 
-[![Language](https://img.shields.io/badge/C%2B%2B-98/11/14/17/20-blue.svg)](https://en.wikipedia.org/wiki/C%2B%2B#Standardization) [![License](https://img.shields.io/badge/license-BSL-blue.svg)](https://opensource.org/licenses/BSL-1.0) [![Build Status](https://github.com/martinmoene/boolean-lite/actions/workflows/ci.yml/badge.svg)](https://github.com/martinmoene/boolean-lite/actions/workflows/ci.yml) [![Build Status](https://travis-ci.org/martinmoene/boolean-lite.svg?branch=master)](https://travis-ci.org/martinmoene/boolean-lite) [![Build status](https://ci.appveyor.com/api/projects/status/1ha3wnxtam547m8p?svg=true)](https://ci.appveyor.com/project/martinmoene/boolean-lite) [![Version](https://badge.fury.io/gh/martinmoene%2Fboolean-lite.svg)](https://github.com/martinmoene/boolean-lite/releases) [![download](https://img.shields.io/badge/latest-download-blue.svg)](https://github.com/martinmoene/boolean-lite/blob/master/include/nonstd/boolean.hpp) [![Conan](https://img.shields.io/badge/on-conan-blue.svg)](https://conan.io/center/boolean-lite) [![Try it on wandbox](https://img.shields.io/badge/on-wandbox-blue.svg)](https://wandbox.org/permlink/TMURDai5p2CSWn9a) [![Try it on godbolt online](https://img.shields.io/badge/on-godbolt-blue.svg)](https://godbolt.org/z/E1onYM)
+[![Language](https://img.shields.io/badge/C%2B%2B-98/11/14/17/20-blue.svg)](https://en.wikipedia.org/wiki/C%2B%2B#Standardization) [![License](https://img.shields.io/badge/license-BSL-blue.svg)](https://opensource.org/licenses/BSL-1.0) [![Build Status](https://github.com/martinmoene/boolean-lite/actions/workflows/ci.yml/badge.svg)](https://github.com/martinmoene/boolean-lite/actions/workflows/ci.yml) [![Build status](https://ci.appveyor.com/api/projects/status/1ha3wnxtam547m8p?svg=true)](https://ci.appveyor.com/project/martinmoene/boolean-lite) [![Version](https://badge.fury.io/gh/martinmoene%2Fboolean-lite.svg)](https://github.com/martinmoene/boolean-lite/releases) [![download](https://img.shields.io/badge/latest-download-blue.svg)](https://github.com/martinmoene/boolean-lite/blob/master/include/nonstd/boolean.hpp) [![Conan](https://img.shields.io/badge/on-conan-blue.svg)](https://conan.io/center/boolean-lite) [![Try it on wandbox](https://img.shields.io/badge/on-wandbox-blue.svg)](https://wandbox.org/permlink/TMURDai5p2CSWn9a) [![Try it on godbolt online](https://img.shields.io/badge/on-godbolt-blue.svg)](https://godbolt.org/z/E1onYM)
 
 **Contents**