Problem:

Bundling the gem for deployment raised the errors:

```
WARNING:  open-ended dependency on autoprefixer-rails (>= 0) is not recommended
  if autoprefixer-rails is semantically versioned, use:
    add_runtime_dependency 'autoprefixer-rails', '~> 0'
WARNING:  open-ended dependency on momentjs-rails (>= 2.9.0) is not recommended
  if momentjs-rails is semantically versioned, use:
    add_runtime_dependency 'momentjs-rails', '~> 2.9', '>= 2.9.0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
```

Solution:

Use optimistic versioning for all dependencies,
under the assumption they're all using semantic versioning.