build boost with graph library even when +mpi

Previously, when +graph and +mpi were enabled, the graph library
was replaced with the graph_parallel library. This alters the
logic for +graph+mpi to build both libraries.

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

@@ -324,7 +324,6 @@ def install(self, spec, prefix):
         if not spec.satisfies('@1.43.0:'):
             withLibs.remove('random')
         if '+graph' in spec and '+mpi' in spec:
-            withLibs.remove('graph')
             withLibs.append('graph_parallel')
 
         # to make Boost find the user-config.jam