chore: Use build status of main branch in README

Author: j-ulrich

README.md

@@ -1,7 +1,7 @@
 # HTTP Status Codes and Reason Phrases for C, C++ and Qt #
 
-[![Linux build status](https://travis-ci.com/j-ulrich/http-status-codes-cpp.svg?branch=master)](https://travis-ci.com/j-ulrich/http-status-codes-cpp)
-[![Windows build status](https://ci.appveyor.com/api/projects/status/ijp196mjo0vsover/branch/master?svg=true)](https://ci.appveyor.com/project/j-ulrich/http-status-codes-cpp/branch/master)
+[![Linux build status](https://travis-ci.com/j-ulrich/http-status-codes-cpp.svg?branch=main)](https://app.travis-ci.com/github/j-ulrich/http-status-codes-cpp)
+[![Windows build status](https://ci.appveyor.com/api/projects/status/ijp196mjo0vsover/branch/main?svg=true)](https://ci.appveyor.com/project/j-ulrich/http-status-codes-cpp/branch/main)
 
 
 This repository provides the HTTP status codes and reason phrases in different variants for C/C++.