Don't explicitly clean before testing (#210)

This no longer appears to be necessary, and even causes problems on sbt 1.3.
See also #209

Author: raboof

Diff for: build.sbt

@@ -22,9 +22,6 @@ lazy val root = (project in file("."))
       result
     },
     test in Test := {
-      // since we are building for different Scala patch versions, a clean
-      // is required to avoid conflicts in class files
-      clean.value
       saveTestClasspath.value
       (test in Test).value
     },