Fix rare crash in tests for travis and fix coverage

.travis.yml

@@ -29,4 +29,4 @@ notifications:
 #    on_failure: always
 
 after_success:
-  - coveralls
+  - coveralls --base_dir gns3server

gns3server/modules/vpcs/__init__.py

@@ -52,6 +52,7 @@ class VPCS(BaseManager):
         vm = self.get_vm(vm_id)
         i = self._used_mac_ids[vm_id]
         self._free_mac_ids[vm.project.id].insert(0, i)
+        if vm_id in self._used_mac_ids:
             del self._used_mac_ids[vm_id]
         yield from super().close_vm(vm_id, *args, **kwargs)
         return vm