Upgrade to Scala 2.12.4

Remove the YdisableFlatCpCaching workaround. The bug has been fixed
in Scala 2.12.

Fixes bazel-contrib#305

Author: gkk-stripe

scala/scala.bzl

@@ -206,10 +206,7 @@ DependencyAnalyzerMode: {dependency_analyzer_mode}
 """.format(
         out=ctx.outputs.jar.path,
         manifest=ctx.outputs.manifest.path,
-        # always append -YdisableFlatCpCaching, workaround for
-        # https://github.com/bazelbuild/rules_scala/issues/305
-        # remove once we upgrade to Scala 2.12.4
-        scala_opts=",".join(ctx.attr.scalacopts + ["-YdisableFlatCpCaching"]),
+        scala_opts=",".join(ctx.attr.scalacopts),
         print_compile_time=ctx.attr.print_compile_time,
         plugin_arg=plugin_arg,
         cp=compiler_classpath,
@@ -1023,9 +1020,9 @@ java_library(
 def scala_repositories():
   native.new_http_archive(
     name = "scala",
-    strip_prefix = "scala-2.12.3",
-    sha256 = "2b796ab773fbedcc734ba881a6486d54180b699ade8ba7493e91912044267c8c",
-    url = "https://downloads.lightbend.com/scala/2.12.3/scala-2.12.3.tgz",
+    strip_prefix = "scala-2.12.4",
+    sha256 = "9554a0ca31aa8701863e881281b1772370a87e993ce785bb24505f2431292a21",
+    url = "https://downloads.lightbend.com/scala/2.12.4/scala-2.12.4.tgz",
     build_file_content = SCALA_BUILD_FILE,
   )