Use merge_db_schema for db/schema.rb conflicts

This Gem can be used to automatically resolve merge conflicts in
db/schema.rb. To use it you still need to initialise the configuration,
which can be done by running:

    merge_db_schema-init --force

See https://gitlab.com/gitlab-org/release/framework/issues/42 for more
information.

.gitattributes

 Dangerfile gitlab-language=ruby
+db/schema.rb merge=merge_db_schema