Remove the require of pry-debugger from the root file

pry-debugger is specified as a development dependency in the gemspec, but is required in the root file (lib/ruby-wpdb.rb). This results in load errors when requiring the gem.

Here's the irb stack trace: 

    > require 'ruby-wpdb'
    LoadError: cannot load such file -- pry-debugger
      from /Users/dxwduncan/.rvm/gems/ruby-1.9.3-p448/gems/ruby-wpdb-1.0/lib/ruby-wpdb.rb:5:in `require'
      from /Users/dxwduncan/.rvm/gems/ruby-1.9.3-p448/gems/ruby-wpdb-1.0/lib/ruby-wpdb.rb:5:in `<top (required)>'
      from (irb):3:in `require'