Switch to official Standard gem

[mrbiggred]

Once this PR is approved and merged into the Standard gem this project should switch from using the Corgibytes fork to the official gem.

Current gemfile looks like:

if RUBY_VERSION >= "2.6"
    gem "standard", git: "https://github.com/corgibytes/standard", ref: "64ddc304a5df706ede8bbbb3b0483cc67ee65a4a"
end