updating GSON and Guava to more recent versions (#8524)

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 064cf90..75853ff 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -38,8 +38,8 @@
 load("@rules_jvm_external//:defs.bzl", "maven_install")
 maven_install(
     artifacts = [
-        "com.google.guava:guava:18.0",
-        "com.google.code.gson:gson:2.7",
+        "com.google.guava:guava:30.1.1-jre",
+        "com.google.code.gson:gson:2.8.6",
         "com.google.errorprone:error_prone_annotations:2.3.2",
         "junit:junit:4.12",
         "org.easymock:easymock:3.2",
@@ -47,8 +47,9 @@
         "com.google.truth:truth:1.1.2",
     ],
     repositories = [
-        "https://jcenter.bintray.com/",
         "https://repo1.maven.org/maven2",
+        "http://mirrors.ibiblio.org/pub/mirrors/maven2",
+        "http://www.ibiblio.net/pub/packages/maven2",
     ],
     # For updating instructions, see: 
     # https://github.com/bazelbuild/rules_jvm_external#updating-maven_installjson
@@ -93,6 +94,6 @@
     actual = "@maven//:com_google_truth_truth",
 )
 
-# For `cc_proto_blacklist_test`.
+# For `cc_proto_blacklist_test` and `build_test`.
 load("@bazel_skylib//:workspace.bzl", "bazel_skylib_workspace")
 bazel_skylib_workspace()