Remove Travis build badge

We no longer use Travis, so this isn't meaningful... And GitHub
already displays the build status, so there's no value in having
this kind of badge at all.

README.md

@@ -1,7 +1,7 @@
 PHP Parser
 ==========
 
-[![Build Status](https://travis-ci.org/nikic/PHP-Parser.svg?branch=master)](https://travis-ci.org/nikic/PHP-Parser) [![Coverage Status](https://coveralls.io/repos/github/nikic/PHP-Parser/badge.svg?branch=master)](https://coveralls.io/github/nikic/PHP-Parser?branch=master)
+[![Coverage Status](https://coveralls.io/repos/github/nikic/PHP-Parser/badge.svg?branch=master)](https://coveralls.io/github/nikic/PHP-Parser?branch=master)
 
 This is a PHP 5.2 to PHP 8.0 parser written in PHP. Its purpose is to simplify static code analysis and
 manipulation.