Fix update_jar.sh for MacOS.

There's no "readlink -f" option.

Change-Id: If6f885e477188a6e2391f72a52bd1dcc712a72a5
diff --git a/update_jars.sh b/update_jars.sh
index 6275100..938db99 100755
--- a/update_jars.sh
+++ b/update_jars.sh
@@ -2,7 +2,11 @@
 
 set -e            # fail on errors
 
-PROG_DIR=$(readlink -f $(dirname "$0"))
+if [[ $(uname) == "Darwin" ]]; then
+  PROG_DIR=$(dirname "$0")
+else
+  PROG_DIR=$(readlink -f $(dirname "$0"))
+fi
 cd "$PROG_DIR"
 
 DRY="echo"        # default to dry mode unless -f is specified