Update refinerycms-i18n.gemspec

I use refinerycms + spree and I have a version conflict:
```
 Bundler could not find compatible versions for gem "rails-i18n":
In Gemfile:
    
   refinerycms-i18n (>= 0) ruby depends on
      rails-i18n (~> 0.6.5) ruby

    spree_i18n (>= 0) ruby depends on
      rails-i18n (0.7.3)
```