Make sure sherlodoc is installed when updating docs

I noticed I had updated docs without `sherlodoc`, which results in not having
the nice search bar with the docs.  This should prevent that from happening
again.

Author: polytypic

update-gh-pages-for-tag

@@ -28,6 +28,8 @@ EOF
   exit 1
 fi
 
+opam install sherlodoc
+
 mkdir $TMP
 cd $TMP