Make check explicitly 'not None'.

The only way r would be 'None' is if a symmetrical reaction (reactants == products)
was found in a library, and hit this check in check_for_existing_reaction that's called by 
make_new_reaction. But that suggests some other problem. So I doubt this test is often needed.
But Matt added it for something.

I can't see any way it would be False, so now we check it's not None.
(as requested in code review)

Author: rwest

rmgpy/rmg/model.py

@@ -1712,8 +1712,8 @@ def add_reaction_library_to_edge(self, reaction_library):
                                       kinetics=rxn.kinetics, duplicate=rxn.duplicate,
                                       reversible=rxn.reversible
                                       )
-            if r and getattr(r.kinetics, 'coverage_dependence', None):
             r, isNew = self.make_new_reaction(rxn)  # updates self.new_species_list and self.new_reaction_list
+            if r is not None and getattr(rxn.kinetics, 'coverage_dependence', None):
                 self.process_coverage_dependence(r.kinetics)
             if not isNew:
                 logging.info("This library reaction was not new: {0}".format(rxn))