Make sure old extension is cleaned up before building the new one

This should happen automatically but this just makes sure that noone
is left with the old extension locally after installing a new update
that requries a newer version of the extension.

shard.yml

@@ -5,7 +5,7 @@ authors:
   - Kevin Robell <[email protected]>
 
 scripts:
-  postinstall: "make extension"
+  postinstall: 'make clean extension'
 
 crystal: '>= 1.10.1'