util: Update to cross-compile for scala 2.12

Problem

We want to be able to cross-compile for scala 2.12 but scoverage
isn't ready yet.

Based on this:
efcc8a3

Which was reverted here:
8e7e04d

Solution

Make scoverage resolve 2.11 artifacts when building for scala
2.12 until 2.12 versions are available.

Result

We should be able to cross-build and this change doesn't appear
to break any downstream projects.

RB_ID=840265

project/Build.scala

@@ -27,6 +27,7 @@ object Util extends Build {
     version := libVersion,
     organization := "com.twitter",
     scalaVersion := "2.11.8",
+    crossScalaVersions := Seq("2.11.8", "2.12.0-M4"),
     // Workaround for a scaladoc bug which causes it to choke on empty classpaths.
     unmanagedClasspath in Compile += Attributed.blank(new java.io.File("doesnotexist")),
     libraryDependencies ++= Seq(
@@ -49,6 +50,22 @@ object Util extends Build {
       "-Xlint:-missing-interpolator"
     ),
 
+    // scoverage automatically brings in libraries on our behalf, but it hasn't
+    // been updated for 2.12 yet[0].  for now, we need to rely on the 2.11 ones
+    // (which seem to work OK)
+    //
+    // [0]: https://github.com/scoverage/sbt-scoverage/issues/126
+    libraryDependencies := {
+      libraryDependencies.value.map {
+        case moduleId: ModuleID
+          if moduleId.organization == "org.scoverage"
+            && scalaVersion.value.startsWith("2.12") =>
+            moduleId.copy(name = moduleId.name.replace(scalaVersion.value, "2.11"))
+        case moduleId =>
+          moduleId
+      }
+    },
+
     // Note: Use -Xlint rather than -Xlint:unchecked when TestThriftStructure
     // warnings are resolved
     javacOptions ++= Seq("-Xlint:unchecked", "-source", "1.8", "-target", "1.8"),