Fixes #1201. Display instruction to overrideScalaVersion

scala-library is currently treated as just one of the library modules.
This means that it’s at the mercy of eviction if some other library
uses newer version of scala-library.
This commit displays a instruction on how to force scalaVersion if
warnScalaVersionEviction flag is true on EvictionWarningOptions.
Merge pull request #1539 from sbt/wip/1201

Fixes #1201. Display instruction to overrideScalaVersion