Version 7.0.15 is the last release of the OC 7.0-branch, there won't be any updates: https://owncloud.org/changelog/#latest7
You should upgrade to a newer version as soon as possible. Starting with owncloud 8.0, you will need php 5.4 and Windows is no longer supported (https://forum.owncloud.org/viewtopic.php?f=31&t=27449). Starting from 8.0, the numbering of major versions has change, so 8.0, 8.1, and 8.2 are all major versions that you must not skip during an upgrade.
Before an upgrade, please read the release notes carefully. For each major release, we have sticky topics with links to release notes and typical problems and pitfalls, e.g.
Always do full backups before an upgrade, there is no undo-procedure.