When upgrading from Fedora 26 to 27, source-to-image cannot be upgraded because Fedora 26 provides newer version than Fedora 27: source-to-image-1.1.7-1.fc26 > source-to-image-1.0.9-7.fc27 It looks like you forgot to build the package for F27.
source-to-image-1.1.7-1.fc27 has been submitted as an update to Fedora 27. https://bodhi.fedoraproject.org/updates/FEDORA-2018-f728248fd1
source-to-image-1.1.7-1.fc27 has been pushed to the Fedora 27 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2018-f728248fd1
source-to-image-1.1.7-1.fc27 has been pushed to the Fedora 27 stable repository. If problems still persist, please make note of it in this bug report.