A new version is available since several months, I cannot find the tarball, however. :/
The tarball would be in snapshots, not releases. https://www.cairographics.org/snapshots/cairo-1.15.8.tar.xz
1.15.x are no stable releases, just 'snapshots' on the way to the next 1.16 major release. I assume these snapshots do not usually make it into the tree.
*** This bug has been marked as a duplicate of bug 581532 ***