Use explicit targets for babel-preset-env

This reduces bundle size by 5 KiB.

dist/babelify-config.js

@@ -2,6 +2,12 @@ const babelifyConfig = {
     presets: [
         ['@babel/preset-env', {
             'useBuiltIns': 'entry',
+            'targets': {
+                'firefox': 50,
+                'chrome': 45,
+                'opera': 32,
+                'safari': 11,
+            },
         }],
     ],
     extensions: '.ts',