Gemspec now depends on 2.1.0.beta

When trying to install master on a 2.1.0.beta store, bundler gives the message:

```
Bundler could not find compatible versions for gem "spree_core":
  In Gemfile:
    spree_static_content (>= 0) ruby depends on
      spree_core (~> 2.0.0) ruby
 
    spree (>= 0) ruby depends on
      spree_core (2.1.0.beta)
```

This should remedy that.