Remove call to #verify_active_connections!

ActiveRecord::Base.verify_active_connections! has been deprecated since
Rails v2.1.0. It was finally removed in Rails 4.

While it is still possible to manually verify connections by iterating
through the connection pool and calling #verify! on each one, we hope
this is no longer required. We will watch production carefully to make
sure no "mysql server has gone away" errors occur.

Change-Id: I1a414238fc8ddd675bdf0815ffe7ee8342dd5fef
Reviewed-on: http://gerrit.causes.com/35649
Reviewed-by: Tony Wooster <tony.wooster@causes.com>
Tested-by: Tom Dooner <tom.dooner@causes.com>