Enable unusedCompileDependenciesTest

[rossabaker]

Fails the build if we declare a dependency we don't use. Prevents us from passing liabilities downstream.

Not enforced on private (e.g. unpublished) projects.

Bonus: stop publishing empty http4s-tests module.

[cb372]

Looks like it fails for only the primary Travis job, where scoverage is enabled:

[warn] mimedb-generator >>> The following libraries are declared in libraryDependencies but are not needed for compilation:
[warn]  - "org.scoverage" %% "scalac-scoverage-runtime" % "1.3.1"

Not sure how to work around that, other than adding support in the plugin for manually excluding certain dependencies as I suggested at cb372/sbt-explicit-dependencies#13 (comment). I'll try and get that implemented in the next few days.

[rossabaker]

We have a PrivateProjectsPlugin that disables scoverage on certain modules. That's where the call to disable the dependency test really belonged. That should make this green even without the new feature, though I think it's a good one to have.