Better handling of warnings from Meteor bundle

Breaking changes:

• The --prelease option has been removed

All output from Meteor to stderr is now treated as a warning. The output is
reported to the user, but the demeteorization process is not exited unless the
command exits with a non-zero code.

This release also corrects an issue where $ meteor --version would include
much more than just the version of Meteor.

Closes #80 and miscellaneous issues reported on the Modulus CLI.