fixes #8791 - pin i18n gem

This is also failing release_nightly_test_deb at the moment.
with ```gem 'i18n', '~> 0.6', '>= 0.6.4'``` I'm getting 0.7.0, with ```gem 'i18n', '< 0.7.0'``` 0.6.11 as desired, no idea why (bundler 1.7.4 from the Debian/jessie package)