Clear Symfony cache after Grunt build

Following a Grunt "build", a developer must manually clear the Symfony cache to ensure the latest Twig (dist) templates are used in production.

We can further automate the build process by adding a new grunt task to clear the Symfony (production) cache at the end of a build.

Essentially just need to execute:

```
vagrant ssh -c "/vagrant/app/console cache:clear --env=prod"
```
