Add maven compiler source and target to pom.xml (#808)

Closes #807

Author: filiphornak

pom.xml

@@ -63,6 +63,8 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
+        <maven.compiler.source>1.8</maven.compiler.source>
+        <maven.compiler.target>1.8</maven.compiler.target>
 
         <scala.compat.version>2.12</scala.compat.version>
         <spark.version>2.4.4</spark.version>