Fix warning message on CentOS environ.

When I try to build hatohol on CentOS environ, following messages are
occured (I'm using Japanese environment):

CacheServiceDBClient.h: In static member function ‘static void*
CacheServiceDBClient::operator new(size_t)’:
CacheServiceDBClient.h:50: 警告: ‘operator new’ must not return NULL
unless it is declared ‘throw()’ (or -fcheck-new is in effect)