Use HEREDOC on strings

[faustinoaq]

See: https://github.com/amberframework/micrate/blob/master/src/micrate/cli.cr#L65

This'd look better with HEREDOC syntax:

  puts <<-MESSAGE
  Micrate will not run these migrations because they may have been written with an older database model in mind.
  You should probably check if they need to be updated and rename them so they are considered a newer version.
  MESSAGE
end

def self.print_help
  puts <<-HELP
  micrate is a database migration management system for Crystal projects, *heavily* inspired by Goose (https://bitbucket.org/liamstask/goose/).
  
  Usage:
      micrate [options] <subcommand> [-h] [subcommand options]
  Commands:
      up         Migrate the DB to the most recent version available
      down       Roll back the version by 1
      to         Migrate exact to the given version (e.g. micrate to -v 20160524162446)
      <version>  Shorthand for 'micrate to' (e.g. micrate 20160524162446)
      redo       Re-run the latest migration
      status     dump the migration status for the current DB
      create     Create the scaffolding for a new migration
      dbversion  Print the current version of the database
      help       Shows this message
  HELP
end