Update rails-i18n.gemspec

```
rails-i18n at /home/myuser/.rvm/gems/ruby-2.1.0@rails403/bundler/gems/rails-i18n-6b****39 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on rails (= 4.0.2, development), (~> 4.0) use:
    add_runtime_dependency 'rails', '= 4.0.2', '~> 4.0'
```
Merge pull request #427 from mibamur/patch-2

Update rails-i18n.gemspec