build-sys: post-release version bump

Per request by flatpak.

Closes: #928
Approved by: alexlarsson

configure.ac

@@ -1,7 +1,7 @@
 AC_PREREQ([2.63])
 dnl If incrementing the version here, remember to update libostree.sym too
 m4_define([year_version], [2017])
-m4_define([release_version], [6])
+m4_define([release_version], [7])
 m4_define([package_version], [year_version.release_version])
 
 AC_INIT([libostree], [package_version], [walters@verbum.org])