Modernise the release instructions

[ennru]

• Use just a master branch?
• Introduce sbt-dynver?

The Travis secrets for ghpages seem broken, the deploy doesn't work from Travis.

[jonas]

I agree that the git-flow based release instructions can be simplified. 👍 to just using a master branch and automatically setting the version in sbt from git using sbt-dynver.

For Travis, I might have revoked the secrets because they were tied to my account.

[jonas]

I can look into using deploy keys to update the ghpages branch.

[ennru]

Thank you chiming in - good to see you around.
I believe it works well enough to run the ghpages publish from sbt manually.