Make GovukError.configure behave as a singleton

This is to prevent any surprising behaviour should the downstream
app decide to call 'GovukError.configure' twice. Prior to this
commit, calling twice would keep all the Raven configuration but
lose any custom configuration.

I then needed to edit the 'govuk_error_spec' configure test to
prevent `double("Configuration")` from leaking between tests. I
also found that the change would cause configuring_spec.rb to
fail, as the initialised GovukError module would no longer be
looking for the SENTRY_CURRENT_ENV variable at the time the test
would run.

Once I changed this test to accommodate the singleton pattern, we
no longer needed the `config.current_environment=` line, which was
only really there to satisfy configuring_spec.rb. The line is
unnecessary as the default behaviour is to delegate that to Raven,
which is already being set via the ENV variable, so we don't need
to create that ourselves. We also no longer need configuring_spec
itself - its behaviour is now tested in govuk_error_spec.

lib/govuk_app_config/govuk_error.rb

@@ -13,6 +13,7 @@ def self.notify(exception_or_message, args = {})
   end
 
   def self.configure
-    yield Configuration.new(Raven.configuration)
+    @configuration ||= Configuration.new(Raven.configuration)
+    yield @configuration
   end
 end

lib/govuk_app_config/govuk_error/configure.rb

@@ -1,6 +1,4 @@
 GovukError.configure do |config|
-  config.current_environment = ENV["SENTRY_CURRENT_ENV"]
-
   config.before_send = proc { |e|
     GovukStatsd.increment("errors_occurred")
     GovukStatsd.increment("error_types.#{e.class.name.demodulize.underscore}")

spec/lib/govuk_error_spec.rb

@@ -22,10 +22,8 @@
 
   describe ".configure" do
     it "configures Raven via the Configuration" do
-      mock_configuration = double("Configuration")
-      allow(GovukError::Configuration).to receive(:new) { mock_configuration }
-      expect(mock_configuration).to receive(:foo)
-      GovukError.configure(&:foo)
+      expect { |b| GovukError.configure(&b) }
+        .to yield_with_args(instance_of(GovukError::Configuration))
     end
   end
 end