OPENDAP/libdap4: Version 3.20.4 for Hyrax 1.16.0

News for version 3.20.4 Fixed memory leaks in the libxml2-based DMR and DDX parsers. Moved more toward C++-11. On CentOS 6 we now build using c++0x while CentOS 7 uses c++11. This means that we can use unique_ptr but not null_ptr, for example. We have found a bug in the code that orders Dimensions, Enumeration definitions and Groups in the DMR. Listing the Groups last makes DMRs that reference Dimensions defined inside Groups fail silently