we no longer use cElementTree egg.

in python 2.5 or later, we can import from xml.etree.cElementTree instead.
version up : groonga-4.0.0, groonga-normalizer-mysql-1.0.6 and mroonga-4.00.
version up : libpng 1.2.51, 1.5.18 and 1.6.9.
version up : firefox 27.0.