fix issue with compiling for scala 2.13.1

[pjfanning]

Possibly relates to scoverage/sbt-scoverage#295

This is the compile issue:

[info] Fast optimizing /Users/pj.fanning/code/scalac-scoverage-plugin/scalac-scoverage-runtime/js/target/scala-2.13/scalac-scoverage-runtime-test-fastopt.js
[error] /Users/pj.fanning/code/scalac-scoverage-plugin/scalac-scoverage-plugin/src/test/scala/scoverage/ScoverageCompiler.scala:102:34: ambiguous reference to overloaded definition,
[error] both value reporter in class ScoverageCompiler of type scala.tools.nsc.reporters.Reporter
[error] and  method reporter in class Global of type => scala.tools.nsc.reporters.FilteringReporter
[error] match expected type ?
[error]   def assertNoErrors() = assert(!reporter.hasErrors, "There are compilation errors")
[error]                                  ^
[error] one error found

[pjfanning]

This is a duplicate of #279