Raised version to 1.5

Citing the gluon v2018.1 release notes:
    This version changes the flash partition layout on some devices (TP-Link
    CPE/WBS 210/510). To avoid upgrade failures, make sure to upgrade to
    Gluon 2017.1.8 or the latest Gluon 2016.2.x (unreleased) before
    installing Gluon 2018.1.

So we need a yet unreleased ffs-1.4 version and have to bump the version
to 1.5 fot a release with gluon v2018.1.