Update debugger to work with Ruby 2.0.0-p353.

When I attempted to `bundle install` the following error occured on OS X 10.9 (Mavericks):

```
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

    /Users/user/.rbenv/versions/2.0.0-p353/bin/ruby extconf.rb 
checking for rb_method_entry_t.called_id in method.h... no
checking for rb_control_frame_t.method_id in method.h... no
checking for rb_method_entry_t.called_id in method.h... no
checking for rb_control_frame_t.method_id in method.h... no
Makefile creation failed
**************************************************************************
No source for ruby-2.0.0-p353 provided with debugger-ruby_core_source gem.
**************************************************************************
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
  --with-opt-dir
  --without-opt-dir
  --with-opt-include
  --without-opt-include=${opt-dir}/include
  --with-opt-lib
  --without-opt-lib=${opt-dir}/lib
  --with-make-prog
  --without-make-prog
  --srcdir=.
  --curdir
  --ruby=/Users/user/.rbenv/versions/2.0.0-p353/bin/ruby
  --with-ruby-dir
  --without-ruby-dir
  --with-ruby-include=${ruby-dir}/include
  --with-ruby-lib
  --without-ruby-lib=${ruby-dir}/


Gem files will remain installed in /Users/user/Development/opensource/monocle/.bundle/gems/debugger-1.6.1 for inspection.
Results logged to /Users/user/Development/opensource/monocle/.bundle/gems/debugger-1.6.1/ext/ruby_debug/gem_make.out

An error occurred while installing debugger (1.6.1), and Bundler cannot continue.
Make sure that `gem install debugger -v '1.6.1'` succeeds before bundling.
```

debugger 1.6.2 did not cause that issue although [the changes between that and 1.6.1](https://github.com/cldwalker/debugger/blob/master/CHANGELOG.md#162) (the version currently locked) seem minor. 

You can now specify which Ruby patchlevel you expect people to use with [Bundler 1.5.0 RC1](http://bundler.io/v1.5/whats_new.html) by the way: 

```ruby
ruby '2.0.0', patchlevel: "353"
```