fix: lock prettier's version

Author: 柴茂源

template/package.json

@@ -23,6 +23,7 @@
     "mpvue-template-compiler": "^1.0.11",
     "portfinder": "^1.0.13",
     "postcss-mpvue-wxss": "^1.0.0",
+    "prettier": "~1.12.1",
     "px2rpx-loader": "^0.1.10",
     "babel-core": "^6.22.1",
     "glob": "^7.1.2",