2018.4

A quick turnaround after 2018.3 to include one main PR: https://github.com/ostreedev/ostree/pull/1508

"switchroot: Ensure /run/ostree-booted is created even without initramfs"

This fixes ostree when booting without an initramfs. Thanks to @akiernan for the
bug report and helping review the fix! I'm working on enhancing the test suite,
which will help in adding some coverage here.

```
Colin Walters (6 PRs, 10 commits)
  Post-release version bump (#1506)
  ci/papr: Fix artifacts (#1507)
  switchroot: Ensure /run/ostree-booted is created even without initramfs (#1508)
  lib/deltas: Squash some GCC maybe-uninitialized warnings (#1510)
  Release 2018.4 (#1512)
  PR: #1509
    tests/payload-link: Just test a single duplicate object
    tests: Small tweaks for local iteration
    tests/installed: Use temporary directories more consistently
    tests/str: Verify standard-test-roles is installed
    tests/str: Rework sysinstall-tests to be an entrypoint
```

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