Downgrade the enforced Maven version to 3.3.8 to allow the Docker build to work again.

Author: adejanovski

pom.xml

@@ -315,8 +315,8 @@
         <version.jdk>1.8</version.jdk>
         <!-- travis of July 2018 was still using _151 -->
         <build.jdk.minimum>1.8.0-151</build.jdk.minimum>
-        <!-- travis of July 2018 was still using maven-3.5.2 -->
-        <maven.version.minimum>3.5.2</maven.version.minimum>
+        <!-- travis of July 2018 was still using maven-3.5.2 but our Docker image for building packages ships with 3.3.8 -->
+        <maven.version.minimum>3.3.8</maven.version.minimum>
         <surefireArgLine></surefireArgLine>
 
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