Bazel: Mark joda-time as test only

Bug: Issue 12974
Change-Id: Ie56114a419904d789845e14b86886d959874b747

external_plugin_deps.bzl

diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index 5ec6933..fc9e606 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -39,6 +39,7 @@
 
     maven_jar(
         name = "joda-time",
+        testonly = 1,
         artifact = "joda-time:joda-time:2.9.9",
         sha1 = "f7b520c458572890807d143670c9b24f4de90897",
     )