Add puma as default appserver instead of passenger

### Changes:
* add support for downloading files from remote repo instead of
  inlining templates

### Testing updates for template
```bash
$ export TEMPLATE_BRANCH=puma && rails new myapp -m https://raw.github.com/thegarage/thegarage-template/puma/template.rb
```

### Follow-up Tasks
* extract all downloadable files to use new `get_file` utility instead of declaring inline resources