Migrate from maven_jar to jvm_maven_import_external to prepare for Bazel 2.0

WORKSPACE

@@ -15,6 +15,7 @@ load("//:protobuf_deps.bzl", "protobuf_deps")
 
 # Load common dependencies.
 protobuf_deps()
+load("@bazel_tools//tools/build_defs/repo:jvm.bzl", "jvm_maven_import_external")
 
 bind(
     name = "python_headers",
@@ -31,9 +32,14 @@ bind(
     actual = "@submodule_gmock//:gtest_main",
 )
 
-maven_jar(
+jvm_maven_import_external(
     name = "guava_maven",
     artifact = "com.google.guava:guava:18.0",
+    artifact_sha256 = "d664fbfc03d2e5ce9cab2a44fb01f1d0bf9dfebeccc1a473b1f9ea31f79f6f99",
+    server_urls = [
+        "https://jcenter.bintray.com/",
+        "https://repo1.maven.org/maven2",
+    ],
 )
 
 bind(
@@ -41,9 +47,14 @@ bind(
     actual = "@guava_maven//jar",
 )
 
-maven_jar(
+jvm_maven_import_external(
     name = "gson_maven",
     artifact = "com.google.code.gson:gson:2.7",
+    artifact_sha256 = "2d43eb5ea9e133d2ee2405cc14f5ee08951b8361302fdd93494a3a997b508d32",
+    server_urls = [
+        "https://jcenter.bintray.com/",
+        "https://repo1.maven.org/maven2",
+    ],
 )
 
 bind(
@@ -51,9 +62,14 @@ bind(
     actual = "@gson_maven//jar",
 )
 
-maven_jar(
+jvm_maven_import_external(
     name = "error_prone_annotations_maven",
     artifact = "com.google.errorprone:error_prone_annotations:2.3.2",
+    artifact_sha256 = "357cd6cfb067c969226c442451502aee13800a24e950fdfde77bcdb4565a668d",
+    server_urls = [
+        "https://jcenter.bintray.com/",
+        "https://repo1.maven.org/maven2",
+    ],
 )
 
 bind(