Use ActiveRecord::Migrator  multiple migrations paths support

[#44034071]

ActiveRecord::Migrator has a class attribute, migrations_paths,
specificially for storing a list of different directories that have
migrations in them.  ActiveRecord::Migrator.migrations_paths is used in
rake db:load_config, which is a dependency of db:migrate, etc. that is
passed to ActiveRecord::Migrator.migrate.  Since migrate supports an
array of directories, and not just a single directory, there is no need
to merge all the migrations paths into one temporary directory as was
previously done.
Correct return type of Msf::DBManager#migrate.
Document Msf::DBManager#initialize_metasploit_data_models
Update Gemfile to metaploit_data_models 0.6.0

[#44034071]
Update gemcache with metasploit_data_models 0.6.0

[#44034071]
Merge pull request #1536 from rapid7/feature/active-record-migrator-migrations-paths

Use ActiveRecord::Migrator  multiple migrations paths support