fix: bableOpts plugins assignment

Author: yeojz

Diff for: lib/build.js

@@ -9,7 +9,7 @@ function webpackConfig(dir, additionalConfig) {
   var babelOpts = {cacheDirectory: true};
   if (!fs.existsSync(path.join(process.cwd(), '.babelrc'))) {
     babelOpts.presets = ["es2015"];
-    babelOptsplugins = [
+    babelOpts.plugins = [
       "transform-class-properties",
       "transform-object-assign",
       "transform-object-rest-spread"