Fix compatibility with elixir 0.14.1

Relates to #2.

@0xAX, it seems we have an have already had an issue where passing the `--grunt` option has not been working. I've upgraded the system around the grunt and generate modules, but we should probably open another issue for that problem.

@niklassaers, please let me know if you have trouble compiling the project `weber-contrib` now by running the following commands:

```bash
git clone https://github.com/elixir-web/weber-contrib.git
cd weber-contrib
git checkout 0.14.1
mix deps.get
mix
```

Once we merge this into master, this change will fix compiling `weber` using elixir 0.14.1.