changing this dependency so it works with Rails 3.2

@radar please take a look.


```Gemfile
gem 'rails', '3.2.14'
gem 'spree'
gem 'spree_i18n', github: 'yakko/spree_i18n'```


after changing this file, the error below no longer shows.

I hope this commit is beneficial to other people in the international spree community who need this gem.


```deployer@debian:~/host/apps/demo_store$ bundle
Fetching git://github.com/spree/spree_i18n.git
remote: Counting objects: 4533, done.
remote: Compressing objects: 100% (2368/2368), done.
remote: Total 4533 (delta 2138), reused 3810 (delta 1496)
Receiving objects: 100% (4533/4533), 5.06 MiB | 66 KiB/s, done.
Resolving deltas: 100% (2138/2138), done.
Fetching gem metadata from https://rubygems.org/.......
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "spree_core":
  In snapshot (Gemfile.lock):
    spree_core (2.0.4)

  In Gemfile:
    spree_i18n (>= 0) ruby depends on
      spree_core (~> 2.1.0.beta) ruby

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.
```