Add .less to be consistent
Merge branch 'master' of github.com:piwik/piwik
fixes #5325 use setImportDir instead of addImportDir so that the default dir "" (empty string) is not used (as it triggers warning on windows with openbasedir)

This fixes error Warning:is_file(): open_basedir restriction in effect. File(/plugins/Zeitgeist/stylesheets/less/_colors.less.less) is not within the allowed path(s): (C:\Inetpub\temp;D:\Inetpub) in D:\Inetpub\WWWRoot\piwik\vendor\leafo\lessphp\lessc.inc.php on line 82