Bumped Spark version in sample application

Author: sapessi

samples/spark/pet-store/pom.xml

@@ -27,7 +27,7 @@
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
         <jackson.version>2.8.5</jackson.version>
-        <spark.version>2.6.0</spark.version>
+        <spark.version>2.7.1</spark.version>
     </properties>
 
     <dependencies>