Unix packaging: Fix UPX detection for binary compression.

As pointed out by Cristian Ciupitu, our UPX detection logic was broken.
This is now fixed by using `type` to look for the executable.

http://code.google.com/p/phantomjs/issues/detail?id=928

deploy/package.sh

@@ -102,7 +102,7 @@ echo "done"
 echo
 
 echo -n "compressing binary..."
-if [[ ! -z upx ]]; then
+if type upx >/dev/null 2>&1; then
     upx -qqq -9 $phantomjs
     echo "done"
 else