Do not override spec-version for new dependency-track users

[prabhu]

Instead of setting a default at yargs level, the code must override to 1.4 or set 1.5 only if the argument --spec-version was not used.

https://github.com/CycloneDX/cdxgen/blob/master/bin/cdxgen.js#L157

This way, users of the upcoming dependency track can use the 1.5 spec.