chore: update libraries-bom to 4.2.0 (#79)

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Synth log will be available here:
https://source.cloud.google.com/results/invocations/badf69d0-35e6-44bc-af51-7c1c8f83a086/targets

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>4.1.1</version>
+      <version>4.2.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>

synth.metadata

@@ -1,20 +1,20 @@
 {
-  "updateTime": "2020-02-24T22:55:38.663430Z",
+  "updateTime": "2020-02-28T20:09:44.359563Z",
   "sources": [
     {
       "generator": {
         "name": "artman",
-        "version": "0.45.1",
-        "dockerImage": "googleapis/artman@sha256:36956ca6a4dc70a59de5d5d0fd35061b050bb56884516f0898f46d8220f25738"
+        "version": "0.46.0",
+        "dockerImage": "googleapis/artman@sha256:6248a21fba8f0d618c3b032669cec2618e684b66922bb70391fe08c41b05f6ee"
       }
     },
     {
       "git": {
         "name": "googleapis",
         "remote": "https://github.com/googleapis/googleapis.git",
-        "sha": "1e47e676cddbbd8d93f19ba0665af15b5532417e",
-        "internalRef": "296901854",
-        "log": "1e47e676cddbbd8d93f19ba0665af15b5532417e\nFix: Restore a method signature for UpdateCluster\n\nPiperOrigin-RevId: 296901854\n\n"
+        "sha": "e9e90a787703ec5d388902e2cb796aaed3a385b4",
+        "internalRef": "297671458",
+        "log": "e9e90a787703ec5d388902e2cb796aaed3a385b4\nDialogflow weekly v2/v2beta1 library update:\n  - adding get validation result\n  - adding field mask override control for output audio config\nImportant updates are also posted at:\nhttps://cloud.google.com/dialogflow/docs/release-notes\n\nPiperOrigin-RevId: 297671458\n\n1a2b05cc3541a5f7714529c665aecc3ea042c646\nAdding .yaml and .json config files.\n\nPiperOrigin-RevId: 297570622\n\ndfe1cf7be44dee31d78f78e485d8c95430981d6e\nPublish `QueryOptions` proto.\n\nIntroduced a `query_options` input in `ExecuteSqlRequest`.\n\nPiperOrigin-RevId: 297497710\n\ndafc905f71e5d46f500b41ed715aad585be062c3\npubsub: revert pull init_rpc_timeout & max_rpc_timeout back to 25 seconds and reset multiplier to 1.0\n\nPiperOrigin-RevId: 297486523\n\nf077632ba7fee588922d9e8717ee272039be126d\nfirestore: add update_transform\n\nPiperOrigin-RevId: 297405063\n\n0aba1900ffef672ec5f0da677cf590ee5686e13b\ncluster: use square brace for cross-reference\n\nPiperOrigin-RevId: 297204568\n\n5dac2da18f6325cbaed54603c43f0667ecd50247\nRestore retry params in gapic config because securitycenter has non-standard default retry params.\nRestore a few retry codes for some idempotent methods.\n\nPiperOrigin-RevId: 297196720\n\n1eb61455530252bba8b2c8d4bc9832960e5a56f6\npubsub: v1 replace IAM HTTP rules\n\nPiperOrigin-RevId: 297188590\n\n80b2d25f8d43d9d47024ff06ead7f7166548a7ba\nDialogflow weekly v2/v2beta1 library update:\n  - updates to mega agent api\n  - adding field mask override control for output audio config\nImportant updates are also posted at:\nhttps://cloud.google.com/dialogflow/docs/release-notes\n\nPiperOrigin-RevId: 297187629\n\n0b1876b35e98f560f9c9ca9797955f020238a092\nUse an older version of protoc-docs-plugin that is compatible with the specified gapic-generator and protobuf versions.\n\nprotoc-docs-plugin >=0.4.0 (see commit https://github.com/googleapis/protoc-docs-plugin/commit/979f03ede6678c487337f3d7e88bae58df5207af) is incompatible with protobuf 3.9.1.\n\nPiperOrigin-RevId: 296986742\n\n"
       }
     },
     {