Fix Pry.run_command

The problem was that Pry#rep() no longer works on its own, it's
necessary to use Pry#repl() instead. This is because many new API, like
Pry::CodeObject.lookup now get the target binding directly from the _pry_ instance
(Pry#current_context), rather than requiring it to be passed in
separately. This saves us a parameter.

The reason Pry#rep doesnt work with Pry::CodeObject.lookup (and so
show-source/show-doc etc) is that the binding_stack is only set up in
Pry#repl_prologue, and it is the binding_stack (via Pry#current_context)
that is accessed by Pry::CodeObject and pals.