Check for a new version available

To prevent people running older versions of convert2rhel, add a check that would compare the version in use with the latest available version from https://ftp.redhat.com/redhat/convert2rhel/.

In case the URL is not accessible, print out just a warning and continue with the conversion (the system may be intentionally disconnected from the internet).

In case a newer version is available:

• Stop the conversion in case of conversions to RHEL 7 and 8 (supported paths).
• Warn when converting to RHEL 6 since we don't care that much about what version people use for unsupported conversions.