Raise our required version to 4.0.6, mainly to make sure DIRECTORY_SEPARATOR
is available but also to make way for using the variable ordered parameters
feature of sprintf() in the internationalization code.

4.0.6 is two years old now so it should be a more than reasonable requirement.

• core.php: remove DIRECTORY_SEPARATOR definition

• core/php_api.php: raise required version to 4.0.6

• core/compress_api.php
  (compress_is_enabled): remove check for PHP greater than 4.0.4 - we now
  require 4.0.6 anyway

• doc/INSTALL:

• doc/README:

• doc/configuration.html:
  update version number in documentation

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1839 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9