Use unsafe_load when loading config to support aliases with psych 4

[jduff]

Psych 4 changes load to safe_load which doesn't allow aliases in the yaml file. To allow aliases you need to call unsafe_load which is what this PR is changing.

This is making similar assumptions to Rails that a file within your project is safe because it is controlled by the project owner. A recent example of how this was fixed in Rails is here

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[rabotyaga]

Hi @jduff 👋 !
Thanks for the PR!