Add use strict on vendor JS files

vendor/download-releases.js

@@ -1,3 +1,5 @@
+'use strict'; // eslint-disable-line
+
 const fetch = require('node-fetch');
 const mkdirp = require('mkdirp');
 const nugget = require('nugget');

vendor/svg_raster.js

@@ -1,3 +1,5 @@
+'use strict'; // eslint-disable-line
+
 const fs = require('pn/fs');
 const iconGen = require('icon-gen');
 const Jimp = require('jimp');