Add pry to both development and test environments

This is a personal tweak to add a debugging tool to the `email-alert-api` for
those who practice Pry Driven Development :)

Create a joint group for gems that use both development and test environments.
Handle a topic already existing on GovDelivery's side.

Previously, when we attempted to create a subscriber list, if the topic already
exists with GovDelivery, the creation would fail "silently" and we'd get a bad
"SubscriberList" record stored in the email-alert-api.

Now, it will recognise that the list already exists, and populate the data
locally (using the list topics API) so that the error condition won't be
encountered again.

In addition, if *any* error condition is encountered with GovDelivery, an
exception will now be raised rather than it being swallowed silently.
Merge pull request #46 from alphagov/create-list-if-not-exists

Create SubscriberList if it exists on GovDelivery but not locally.