Update version of i18n

I want to embed my middleman-extension `middleman-presentation` into a rails-app. Rails requires `i18n` to be `0.7.0`. `middleman` requires it to be `~>0.6.9`. This PR upgrades the version. 

I ran the test suite and no test failed because of this upgrade. When running `git diff v0.6.9..v07.0` on the the [repository](https://github.com/svenfuchs/i18n/compare/v0.6.9...v0.7.0) it looks more like a minor release.

 Along with this change I discovered some minor `rubocop`-offenses I will fix in a separate PR. I would be happy, if you merge this change and release a new minor version with it. :-) Thanks.