Building error on travis ci https://travis-ci.org/cocos2d/cocos2d-x/jobs/7268093
<pre>
../CCConfiguration.cpp: In member function ‘double cocos2d::CCConfiguration::getNumber(const char*) const’:

../CCConfiguration.cpp:238:9: error: converting to non-pointer type ‘double’ from NULL [-Werror=conversion-null]

cc1plus: all warnings being treated as errors
</pre>

We should return 0.0 instead.