bump the safe_yaml gem

`safe_yaml` version 0.9.4 is the [cause of a travis failure](https://travis-ci.org/alphagov/feedback/builds/9935706), and googling seems to point to [incorrect file perms in the gem as the culprit](https://github.com/dtao/safe_yaml/issues/43). This issue is fixed in `safe_yaml` v0.9.5