Fix #148: Add State(Enum) to social_graph_snippets.py (#167)

solutions/system_design/social_graph/social_graph_snippets.py

 # -*- coding: utf-8 -*-
 from collections import deque
+from enum import Enum
+
+
+class State(Enum):
+    unvisited = 0
+    visited = 1
 
 
 class Graph(object):