Support scoverage 1.0

The underlying scalac plugin has changed; this introduces a breaking change to the gradle plugin.

Also includes an attempt to determine a build version from the nearest git tag (should simplify the release process)
Good spot! Interestingly enough, no - I shouldn't have added the runtime dependency at all, it's not required here. I think I accidentally edited this file when I meant to edit the gradle build that is used for testing...

The plugin is compiled against a version of the scalac plugin, but then doesn't expose this dependency downstream; this is the reasoning behind not adding it directly to the compile configuration. 

In hindsight I wonder if it would be helpful to provide a default configuration rather than forcing users to state the scoverage version.

I'm going to try to rebase this pull request, so this comment may disappear, or I may have to start a new one...