Revert "Parse `gem env` output as yaml instead of using #val_for."

The `gem env` output is not always parseable by psych, e.g.:

    - "gem" => "--no-rdoc --no-ri"

This reverts commit 934ba9112bc73b1adf8cb54588af8099db3f3fc1.
v0.17.3 - Revert the YAML change in CurrentRuby, which some gem
configs can break.

That change was to support multiple gem paths, which isn't implemented
yet; I'll work out a different way of achieving that.

Thanks to James Healy for reporting this.