Remove explicit HDF5 dependency from hdf5 rockspec.

This causes problems unnecessarily on some platforms, where luarocks is unable
to find HDF5 even though it's installed and CMake can find it just fine. We can
avoid the trouble by removing the dependency from the rockspec and relying on
CMake to tell the user if HDF5 is missing.
Merge pull request #51 from d11/master

Remove explicit HDF5 dependency from hdf5 rockspec.