[GEM] Depend on Slop "~> 3.4"

Slop v3.4.0 includes new syntax for subcommands. And we're going to
utilise it.
Remove old subcommands cruft

  * use a plain old Slop instance instead of `Options`;
  * remove `ClassCommand::Options` and its tests;
  * add clearer subcommand example (for `Command#subcommand` method);
  * refactor `ClassCommand#slop`;
  * refactor `ClassCommand#complete`.

Slop v3.4.0 has introduced full-featured subcommands. There is no need
in Pry specific code anymore.
Update relevant tests in "command_spec.rb"

Make them use the new Slop syntax.
Update examples for ClassCommand#subcommands
Update hist command to the new Slop API

It was creating a new Slop instance for its own needs, thus and so
remove the previous abstraction for Slop commands. The
`ClassCommand::Options` class should be removed because we don't need it
anymore.