Use Java 8 for data transfer sample.

Author: tswast

bigquery/datatransfer/cloud-client/pom.xml

@@ -28,8 +28,8 @@
   </parent>
 
   <properties>
-    <maven.compiler.target>1.7</maven.compiler.target>
-    <maven.compiler.source>1.7</maven.compiler.source>
+    <maven.compiler.target>1.8</maven.compiler.target>
+    <maven.compiler.source>1.8</maven.compiler.source>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
   </properties>