i18n.md: Remove Rails versions from Traco link

It is not intended to be limited to only Rails 3 and 4, and currently runs specs against Rails 4.2 to to 5.2: https://github.com/barsoom/traco/blob/master/.travis.yml

guides/source/i18n.md

@@ -1207,7 +1207,7 @@ Several gems can help with this:
 
 * [Globalize](https://github.com/globalize/globalize): Store translations on separate translation tables, one for each translated model
 * [Mobility](https://github.com/shioyama/mobility): Provides support for storing translations in many formats, including translation tables, json columns (PostgreSQL), etc.
-* [Traco](https://github.com/barsoom/traco): Translatable columns for Rails 3 and 4, stored in the model table itself
+* [Traco](https://github.com/barsoom/traco): Translatable columns stored in the model table itself
 
 Conclusion
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