Update tycho to 0.20.0, otherwise JavaSE-1.8 is not recognized as an execution environment.

fortiss-std-env/maven-build/master-pom.xml

@@ -42,7 +42,7 @@
 	    	<plugin>
 	          <groupId>org.eclipse.tycho</groupId>
 	          <artifactId>tycho-surefire-plugin</artifactId>
-	          <version>0.18.0</version>
+	          <version>0.20.0</version>
 	        </plugin>
     		<!--This plugin's configuration is used to store Eclipse m2e settings only. It has no influence on the Maven build itself.-->
     		<plugin>