fix out of memory in travis

Author: ravenq

package.json

@@ -23,7 +23,7 @@
     "serve": "vue-cli-service serve ./example/main.js",
     "build": "npm run build:lib && npm run build:example",
     "lint": "vue-cli-service lint",
-    "build:example": "node --max_old_space_size=8192 node_modules/@vue/cli-service/bin/vue-cli-service.js --dest example/dist ./example/main.js",
+    "build:example": "node --max_old_space_size=8192 node_modules/@vue/cli-service/bin/vue-cli-service.js build --dest example/dist ./example/main.js",
     "build:lib": "node --max_old_space_size=8192 node_modules/@vue/cli-service/bin/vue-cli-service.js build --target lib ./src/index.js"
   },
   "main": "dist/markdown-it-vue.umd.min.js",