Publishing a release

These are the rough steps for publishing a new version of Spectrum.

0. Make sure all changes you want are merged into the master branch.
1. Create a new branch off master for the version bump, e.g. release-x.y.z
2. Update the stableVersion property in gradle.properties to be x.y.z
3. Run ./gradlew build to re-generate the docs and check that everything works.
4. Create a pull request from the branch. This runs a build on Travis CI.
5. When everything looks good and runs green, create an annotated tag in Git to mark the official version. git tag -a x.y.z -m "Release version x.y.z" (tags should just be semver numbers, no v prefix or anything.)
6. Push the tag to GitHub (git push origin x.y.z). This triggers another Travis CI build, which will publish the release artifacts. Publishing only happens on annotated tag builds.
7. Update the release notes on GitHub to provide some info about what is new/changed/fixed in this new version.
8. Wait for the release to fully propagate JCenter and Maven Central. Otherwise the installation instructions would point to a version isn't available for download yet.
9. Merge the pull request.