Make update_rs_prebuilts.sh work on darwin15 (OSX El Capitan)

Bug: 27391260
Change-Id: I85fc7400a713b79a96d4a5df3d2ae42b0565fe8d
diff --git a/update_rs_prebuilts.sh b/update_rs_prebuilts.sh
index 62e4d19..78906d6 100755
--- a/update_rs_prebuilts.sh
+++ b/update_rs_prebuilts.sh
@@ -4,7 +4,7 @@
 MY_ANDROID_DIR=$PWD/../../
 cd $MY_ANDROID_DIR
 
-if [ $OSTYPE == 'darwin13' ] || [ $OSTYPE == 'darwin14' ];
+if [[ $OSTYPE == darwin* ]];
 then
 
   DARWIN=1