Bump release for 2017.5
Bump release for 2017.5

This commit won't actually *be* 2017.5 since due to the way our infrastructure
works, we still want to increment git master to 2017.5.

See https://github.com/ostreedev/ostree/pull/800

Closes: #800
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree