Removes prerelease and treats `stderr` as warnings

This is a breaking change. Includes the removal of the `--prerelease`
flag as `stderr` output from the bundle command is now treated as a
warning. Actual errors will exit with an Error in the callback which
will exit, but output to stderr will just be output.

Related to #78.
Updated the createTarball method

Prevents stderr from failing the demeteorization process. Added test
case to verify that nothing is broken from this change.
Merge pull request #81 from onmodulus/feature/errors

Removes prerelease option and treats stderr as warnings