audit: "version should not decrease" check ignores version_scheme

[zmwangx]

Sample false positive: Homebrew/homebrew-core#6863.

Code in question from audit.rb:

    versions = attributes_map[:version].values.flatten
    if !versions.empty? && formula.version < versions.max
      problem "version should not decrease"
    end

When version_scheme was bumped at some point, e.g., in the case of pyenv, versions will contain:

1.0.2
1.0.2
1.0.1
1.0.0
20160726
20160629
20160628
20160509
20160422
20160310

and a new version number like 1.0.3 will be smaller than versions.max.

[zmwangx]

Probably the proper solution is to compare against the latest version (including the version scheme), not all versions. Homebrew/homebrew-core#6835 (comment) which @ilovezfs brought to my attention should also be resolved that way.