Database migration do not execute when application is run as a rubygem

Due to testing of containerezed Herald: https://github.com/coi-gov-pl/docker-herald the issue arise that database migrations do not execute when installed as a rubygem in system. This is probably due to relative path used in https://github.com/wavesoftware/gem-puppet-herald/blob/develop/lib/puppet-herald/app/configuration.rb#L57

Thanks for bug report @Cosaquee
