Scoverage support for scala 2.11

[myroslavlisniak]

I tried to run pantsbuild 1.19.0 with enabled scoverage for scala 2.11 with command:
./pants --scala-version="2.11" --scoverage-enable-scoverage=False ${TARGET}.

But compilation was failed due to missing compiler plugin.
Scala compiler was invoked with compiler plugin for scala 2.12
-Xplugin:/Users/myroslav.lisniak/projects/poly/.pants.d/resolve/coursier/e2197a99ff41/cache/relative/https/repo1.maven.org/maven2/com/twitter/scoverage/scalac-scoverage-plugin_2.12/1.0.1-twitter/scalac-scoverage-plugin_2.12-1.0.1-twitter.jar

Also I found that plugin's scala version is hardcoded here https://github.com/pantsbuild/pants/blob/master/src/python/pants/backend/jvm/subsystems/scoverage_platform.py#L43