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 0.9.5
Merge pull request #63 from alphagov/bump-safe_yaml_gem

bump the safe_yaml gem