Set default log level to WARN (or TRACE, for verbose)

This commit will result in a cleaner console output by setting the default log level to 'WARN' (2) instead of `TRACE` (0). If the client runs `grunt requirejs` with the `--verbose` flag, the default logLevel will be `TRACE` (0).

I believe this will better match a user's expectation, and will be more in line with the default console output of other grunt tasks.