Switch to Scala nightly build with classpath caching fix

Switch to a recent nightly build that has a fix for the flat classpath
caching bug that is breaking bazel's incremental compilation when worker
is enabled.

See bazel-contrib#251 (comment)
for the details of the bug.

Author: gkk-stripe

scala/scala.bzl

@@ -1027,9 +1027,9 @@ 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-bin-17496bd",
+    sha256 = "b01b4961ab74c6fec46ac783d2e28d350ced1d20d5d74f642c0b68903ab34798",
+    url = "http://www.scala-lang.org/files/archive/nightly/2.12.x/scala-2.12.4-bin-17496bd.tgz",
     build_file_content = SCALA_BUILD_FILE,
   )