Add refinerycms-i18n to gemfile template for engines

[CJYate]

To fix issue #2835

This adds the dependency to an engine's Gemfile.

Manually tested with a dummy app.

[simi]

@parndt should be this locked to stable release once new stable release will be out?

Also @CJBrew can you use new hash syntax?

[CJYate]

Tweaked the hash syntax. Thanks.

[parndt]

@simi yes absolutely, though currently it breaks out of the box so this seems good. @CJBrew Do we need this for forms as well?

[CJYate]

@parndt yes, I think it'll be needed on Forms.

But I've not attempted to merge the work I did on forms generator up to Edge, so there's currently not even a Gemfile to add it to.

[parndt]

No longer required because of #2846