Update python requirement from python 2.4 to python 2.6

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@59009 15284696-431f-4ddb-bdfa-cd5b030d7da7

REQUIREMENTS.html

@@ -130,7 +130,7 @@ Server, <b>Mesa-3.x</b> (openGL clone) may be required for wxNVIZ<br>
   <a href="http://freetype.sourceforge.net">http://freetype.sourceforge.net</a>
 </li>
 
-<li><b>Python &gt;= 2.4</b> (for wxGUI and ctypes interface)<br>
+<li><b>Python &gt;= 2.6</b> (for wxGUI and ctypes interface)<br>
 <i>Note:</i> Python 3 is not supported<br>
 <a href="http://www.python.org">http://www.python.org</a>
 </li>