Remove `tomlrb` as a project dependency?

[fnordfish]

Motivation

As a project-author,
I want to skip Tomlrb as a dependency,
so that the number of dependencies of my gem are minimal.

Rationale:

• My in-house project is fine using YAML or JSON config files.
• My project already uses an a toml parser other than tomlrb.

Specification

• remove s.add_dependency "tomlrb" from mixlib-config.gemspec

Downstream Impact

• Projects that want to use use Toml configs via the build-in from_toml or from_file need to add a dependency on tomlrb by themself.