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)
Merge pull request #44 from greendog/patch-1

Bump rails-i18n version