Fixup the generator script for GNU sed.

GNU sed doesn't understand `-i ''` as meaning don't make a backup
file, wheras BSD sed (as in use on MacOS) doesn't understand `-i`
which GNU sed would use.

Change this to `-i.bak` which works on both sed flavors and delete the
backup file after by hand.

Author: ncw

generator/generate-sdk.sh

@@ -20,6 +20,7 @@ pushd ${spec_dir}
 spec_version=$(git rev-parse --short HEAD)
 popd
 
-sed -i '' -e "s/UNKNOWN SDK VERSION/${sdk_version}/" \
-	-e "s/UNKNOWN SPEC VERSION/${spec_version}/" ${gen_dir}/sdk.go
+sed -i.bak -e "s/UNKNOWN SDK VERSION/${sdk_version}/" \
+    -e "s/UNKNOWN SPEC VERSION/${spec_version}/" ${gen_dir}/sdk.go
+rm ${gen_dir}/sdk.go.bak
 goimports -l -w ${gen_dir}