Add support for APP_VERSION env variable
Add support for APP_VERSION env variable

CLA uses the non-standard APPVERSION env variable. If it doesnt exist,
calls to ping will break the application. This change supports the
standardised version.

ministryofjustice/cla_public,ministryofjustice/cla_public,ministryofjustice/cla_public,ministryofjustice/cla_public