Fix uninitialized constant MarkdownLint::CLI::Pathname at launch
Fix uninitialized constant MarkdownLint::CLI::Pathname at launch

Running mdl <file> fails with:

    /usr/local/lib/ruby/gems/2.3.0/gems/mdl-0.3.0/lib/mdl/cli.rb:155:in `probe_config_file': uninitialized constant MarkdownLint::CLI::Pathname (NameError)
    	from /usr/local/lib/ruby/gems/2.3.0/gems/mdl-0.3.0/lib/mdl/cli.rb:98:in `run'
    	from /usr/local/lib/ruby/gems/2.3.0/gems/mdl-0.3.0/lib/mdl.rb:14:in `run'
    	from /usr/local/lib/ruby/gems/2.3.0/gems/mdl-0.3.0/bin/mdl:10:in `<top (required)>'
    	from /usr/local/bin/mdl:23:in `load'
    	from /usr/local/bin/mdl:23:in `<main>'

It comes from a missing import.

DavidAnson/markdownlint-ruby,markdownlint/markdownlint,mivok/markdownlint