Workaround to avoid module "./htmlparser" not found error in "html-minifier".

Dependency module `html-minifier` sometimes outputs an error during browserify transform due to its use of an incomplete distribution file (`node_modules/html-minifier/dist/all.js`):

```
Error: module "./htmlparser" not found from "./node_modules/rhtml/node_modules/html-minifier/dist/all.js"
```

This PR fixes the above issue by directly requiring actual source file.