HDF5 Dependency: Rewrite to Union

@adamjstewart
```
think you'll find that if you try running something like:
spack spec libsplash ^hdf5@1.8.15
It will complain that libsplash does not depend on hdf5.
This is a bug in Spack's dependency resolution. A workaround
for this is to tell it to always depend on hdf5.
```

@davydden
```
to expand on @adamjstewart comment, spack will make a union
of dependencies,
i.e. hdf5@1.8.6: + hdf5+mpi = hdf5:1.8.6:+mpi, that's why it works.
```

thank you for the hint!

var/spack/repos/builtin/packages/libsplash/package.py

@@ -46,8 +46,8 @@ class Libsplash(Package):
             description='Enable parallel I/O (one-file aggregation) support')
 
     depends_on('cmake', type='build')
-    depends_on('hdf5@1.8.6:', when='~mpi')
-    depends_on('hdf5@1.8.6:+mpi', when='+mpi')
+    depends_on('hdf5@1.8.6:')
+    depends_on('hdf5+mpi', when='+mpi')
     depends_on('mpi', when='+mpi')
 
     def install(self, spec, prefix):