pin grpc-gcp version

Author: rahul2393

google-cloud-spanner/pom.xml

@@ -166,6 +166,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>grpc-gcp</artifactId>
+      <version>1.7.0</version>
     </dependency>
     <dependency>
       <groupId>io.grpc</groupId>