Fix bad use of context manager in git-cookie-authdaemon.

urlopen objects aren't context managers, using them in a with statement
requires wrapping with contextlib.closing.
diff --git a/git-cookie-authdaemon b/git-cookie-authdaemon
index e43eb1e..85e96fd 100755
--- a/git-cookie-authdaemon
+++ b/git-cookie-authdaemon
@@ -8,6 +8,7 @@
 """
 
 import atexit
+import contextlib
 import cookielib
 import json
 import os
@@ -41,7 +42,7 @@
 def acquire_token(retry):
   while True:
     try:
-      with urllib2.urlopen(A + '?scopes=' + S) as token:
+      with contextlib.closing(urllib2.urlopen(A + '?scopes=' + S)) as token:
         return json.load(token)
     except urllib2.URLError:
       if not retry: