Checkstyle license check depends on file in Github master branch 

For Checkstyle plugin we reference file from our master branch. 
  
```
 <headerLocation>https://raw.github.com/codehaus-plexus/plexus-pom/master/src/main/resources/config/plexus-header.txt</headerLocation>
```  
If we remove this file - we may break existing builds 