Update rules_closure to latest version

The latest version removed Java 7 compatibility mode [1] for
protobuf_java rule.

Note that this above fix is only related to the building protobuf_java
rule itself, and is not related to the building of java_proto_library,
that is also affected, but for another reason: default java toolchain
in currently used java_tools distribution is also targeting Java
language level 7 for proto rules. Luckily my PR was accepted here and
Java 7 compatibility mode was removed. However, to take effect a new
java_tools version must be released and Bazel must be updated to point
to it. This will be done in the follow-up change.

[1] https://github.com/bazelbuild/rules_closure/pull/432

Bug: Issue 11102
Change-Id: I0860772dbf78a397f1f8f78f17eb8dcec4ac07c0
diff --git a/WORKSPACE b/WORKSPACE
index 8240f04..2530dd4 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -30,9 +30,9 @@
 
 http_archive(
     name = "io_bazel_rules_closure",
-    sha256 = "eecd37c0eec79e12652c70f2d2e120623cba64616d759ddcedb19e614df618fa",
-    strip_prefix = "rules_closure-d53c0d7755426349d3c443eea4aeedbda27a11be",
-    urls = ["https://github.com/bazelbuild/rules_closure/archive/d53c0d7755426349d3c443eea4aeedbda27a11be.tar.gz"],
+    sha256 = "0409f8bd2a8b6fd1db289cdc0acb394dafd69f60a86d0169bc6495e648e01587",
+    strip_prefix = "rules_closure-18f8acf24ae0d03a9c3ee872ff91dcfbf383d69e",
+    urls = ["https://github.com/bazelbuild/rules_closure/archive/18f8acf24ae0d03a9c3ee872ff91dcfbf383d69e.tar.gz"],
 )
 
 # File is specific to Polymer and copied from the Closure Github -- should be