Remove gelf and graylog2_exceptions gems

More unused gems
Don't auto require rarely used gems

In general it's best to explicitly require libraries rather than expect
bundler to automatically require them. That improves the app's start time and
makes the dependencies more explicit.
Remove colorize, null_logger, gds-warmup-controller

As far as I can see none of these gems are used any more.
Don't load exception_notification in development

Instead we explicitly require it in the initializers that are added on deployment.
Statsd is already required where it's needed
We only use whenever on deployment so don't require it in development
Merge pull request #65 from alphagov/christmas-2012-gem-clearout

Christmas 2012 gem clearout