[ci] Use Maven 3.6.3 on Travis platform.

Signed-off-by: Stéphane Galland <galland@arakhne.org>

.travis.yml

@@ -8,7 +8,7 @@ sudo: required
 # Environment variables for all the OS
 env:
   global:
-    - MAVEN_VERSION: "3.6.2"
+    - MAVEN_VERSION: "3.6.3"
     - MAVEN_OPTS: "-XX:+CMSClassUnloadingEnabled -Xmx3g" 
     - MAVEN_COMPILATION_OPTS: "\"-e -P!generateproduct -Dmaven.test.skip=true -Dio.sarl.docs.tests.skip=true\""
     - MAVEN_TEST_OPTS: "\"-e -P!generateproduct\""