Adding --once argument to test-packages command

So that meteor fails if there is an error and not waits for file change: https://github.com/meteor/meteor/issues/1189#issuecomment-25072673
Forward mrt exit code to make
Support specifying which packages should be tested
Updated README with environment variable
Fix.
Correctly detect when Meteor is ready for testing.

See https://github.com/meteor/meteor/issues/1884 for more details.
need mrt update to ensure to install dependant packages
Allow running custom script for testing
Changed meteor install url to packaging preview