FIX: ensure quantecon package is installed (#113)

Author: mmcky

lectures/chang_credible.md

@@ -30,7 +30,7 @@ In addition to what's in Anaconda, this lecture will need the following librarie
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 tags: [hide-output]
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-!pip install polytope
+!pip install polytope quantecon
 ```
 
 ## Overview