Change this line:
gzip_types text/plain application/x-javascript text/xml text/css;
To be this:
gzip_types text/plain application/javascript application/x-javascript text/javascript text/xml text/css;
Note the addition of application/javascript and text/javascript to your list of gzip types.
There are also more details—and a more expansive list of gzip types—in the answer posted here.