fix: Fix ES6 transpilation (#49)

New versions of Babel don't load any settings by default, which left ES6 in our output.

This puts explicit settings in Babel to transpile down to ES5.

This allows the module to be loaded correctly on older devices, such as our lab's Tizen TV.

Closes #48

package.json

@@ -18,7 +18,7 @@
     "test": "npm run lint && npm run closure-compiler-check && npm run build",
     "lint": "eslint index.js",
     "closure-compiler-check": "google-closure-compiler -O ADVANCED --generate_exports --js_output_file /dev/null --jscomp_error '*' closure-compiler-check/* index.js",
-    "build": "browserify index.js -s EncryptionSchemePolyfills -t babelify -t stripify -p tinyify -p browserify-header -o dist/eme-encryption-scheme-polyfill.js",
+    "build": "browserify index.js -s EncryptionSchemePolyfills -t [ babelify --presets [ @babel/preset-env ] ] -t stripify -p tinyify -p browserify-header -o dist/eme-encryption-scheme-polyfill.js",
     "prepublishOnly": "npm test"
   },
   "devDependencies": {