Remove use of maven_jar.

This is no longer supported in bazel 2:

bazelbuild/bazel#6799

WORKSPACE

@@ -4,6 +4,7 @@
 #
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+load("@bazel_tools//tools/build_defs/repo:jvm.bzl", "jvm_maven_import_external")
 
 # bazel-skylb 0.8.0 released 2019.03.20 (https://github.com/bazelbuild/bazel-skylib/releases/tag/0.8.0)
 skylib_version = "0.8.0"
@@ -49,14 +50,18 @@ bind(
 # junit:junit:4.12
 
 
-# junit:junit:jar:4.12
-maven_jar(
+jvm_maven_import_external(
     name = "org_hamcrest_hamcrest_core",
     artifact = "org.hamcrest:hamcrest-core:1.3",
-    sha1 = "42a25dc3219429f0e5d060061f71acb49bf010a0",
+    artifact_sha256 = "66fdef91e9739348df7a096aa384a5685f4e875584cce89386a7a47251c4d8e9",
+    server_urls = ["http://central.maven.org/maven2"],
+    licenses = ["notice"],
 )
 
-maven_jar(
+jvm_maven_import_external(
     name = "junit_junit",
     artifact = "junit:junit:4.12",
+    artifact_sha256 = "59721f0805e223d84b90677887d9ff567dc534d7c502ca903c0c2b17f05c116a",
+    server_urls = ["http://central.maven.org/maven2"],
+    licenses = ["notice"],
 )