Update sbt to 1.5.6

[scala-steward]

Updates org.scala-sbt:sbt from 1.5.5 to 1.5.6.
GitHub Release Notes - Version Diff

I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.

If you'd like to skip this version, you can just close this PR. If you have any feedback, just mention me in the comments below.

Configure Scala Steward for your repository with a .scala-steward.conf file.

Have a fantastic day writing Scala!

Files still referring to the old version number

The following files still refer to the old version number (1.5.5).
You might want to review and update them manually.

.travis.yml

Ignore future updates

Add this to your .scala-steward.conf file to ignore future updates of this dependency:

updates.ignore = [ { groupId = "org.scala-sbt", artifactId = "sbt" } ]

labels: library-update, early-semver-patch, semver-spec-patch, old-version-remains, commit-count:1

[scala-steward]

Superseded by #394.