Changed Travis to 10.5

Changed Travis status to show status of branch 10.5 (it was still pointing to 10.4)

README.md

 Code status:
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-* [![Travis CI status](https://secure.travis-ci.org/MariaDB/server.png?branch=10.4)](https://travis-ci.org/MariaDB/server) travis-ci.org (10.4 branch)
+* [![Travis CI status](https://secure.travis-ci.org/MariaDB/server.png?branch=10.5)](https://travis-ci.org/MariaDB/server) travis-ci.org (10.5 branch)
 * [![Appveyor CI status](https://ci.appveyor.com/api/projects/status/4u6pexmtpuf8jq66?svg=true)](https://ci.appveyor.com/project/rasmushoj/server) ci.appveyor.com
 
 ## MariaDB: drop-in replacement for MySQL