Remove Travis CI configuration and badge (#6, thanks @striezel)

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

[skip ci]

README.md

@@ -1,6 +1,6 @@
 # bit-lite - C++20 bit operations for C++98 and later in a single-file header-only library.
 
-[![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/bit-lite/actions/workflows/ci.yml/badge.svg)](https://github.com/martinmoene/bit-lite/actions/workflows/ci.yml) [![Build Status](https://travis-ci.org/martinmoene/bit-lite.svg?branch=master)](https://travis-ci.org/martinmoene/bit-lite) [![Build status](https://ci.appveyor.com/api/projects/status/gpmw4gt271itoy2n?svg=true)](https://ci.appveyor.com/project/martinmoene/bit-lite) [![Version](https://badge.fury.io/gh/martinmoene%2Fbit-lite.svg)](https://github.com/martinmoene/bit-lite/releases) [![download](https://img.shields.io/badge/latest-download-blue.svg)](https://raw.githubusercontent.com/martinmoene/bit-lite/master/include/nonstd/bit.hpp) [![Conan](https://img.shields.io/badge/on-conan-blue.svg)](https://conan.io/center/bit-lite) [![Try it online](https://img.shields.io/badge/on-wandbox-blue.svg)](https://wandbox.org/permlink/ZwnY4c9dZUAynUac) [![Try it on godbolt online](https://img.shields.io/badge/on-godbolt-blue.svg)](https://godbolt.org/z/1IPvn7)
+[![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/bit-lite/actions/workflows/ci.yml/badge.svg)](https://github.com/martinmoene/bit-lite/actions/workflows/ci.yml) [![Build status](https://ci.appveyor.com/api/projects/status/gpmw4gt271itoy2n?svg=true)](https://ci.appveyor.com/project/martinmoene/bit-lite) [![Version](https://badge.fury.io/gh/martinmoene%2Fbit-lite.svg)](https://github.com/martinmoene/bit-lite/releases) [![download](https://img.shields.io/badge/latest-download-blue.svg)](https://raw.githubusercontent.com/martinmoene/bit-lite/master/include/nonstd/bit.hpp) [![Conan](https://img.shields.io/badge/on-conan-blue.svg)](https://conan.io/center/bit-lite) [![Try it online](https://img.shields.io/badge/on-wandbox-blue.svg)](https://wandbox.org/permlink/ZwnY4c9dZUAynUac) [![Try it on godbolt online](https://img.shields.io/badge/on-godbolt-blue.svg)](https://godbolt.org/z/1IPvn7)
 
 **Contents**