Revert "Add byebug, for interactive debugging under Ruby 2"

This reverts commit 3b86cb6129140fc123dc3aeffcccdb5652f19085.

Conflicts:
	Gemfile

The problem with this is the combination of these things:

* bundler's behaviour on seeing a platform that it doesn't understand
  is to error, rather than to ignore it:

     https://github.com/bundler/bundler/issues/2428

* mySociety's servers currently only have version 1.1.4 of bundler

* The :ruby_20 platform was only introduced in version 1.3.0 of
  bundler:

     https://github.com/bundler/bundler/blob/master/CHANGELOG.md#130pre3-dec-21-2012

So, for the moment we're just going to revert this change;
upgrading bundler on our servers is a non-trivial job and being
able to deploy on our servers is urgent and important.