bump proto version (#3718)

Author: yihanzhen

google-api-grpc/pom.xml

@@ -95,7 +95,7 @@
       <dependency>
         <groupId>com.google.protobuf</groupId>
         <artifactId>protobuf-java</artifactId>
-        <version>3.6.0</version>
+        <version>3.6.1</version>
       </dependency>
       <dependency>
         <groupId>io.grpc</groupId>

google-cloud-clients/pom.xml

@@ -162,7 +162,7 @@
     <nettyssl.version>2.0.7.Final</nettyssl.version>
     <guava.version>20.0</guava.version>
     <http-client.version>1.24.1</http-client.version>
-    <protobuf.version>3.6.0</protobuf.version>
+    <protobuf.version>3.6.1</protobuf.version>
     <opencensus.version>0.15.0</opencensus.version>
     <!-- test dependency versions -->
     <easymock.version>3.4</easymock.version>