Use Java 7 JDK for all released Maven versions and 8 for planned 3.7.x

- to fix issue with jdk/maven matrix for plugin testing
diff --git a/vars/jenkinsEnv.groovy b/vars/jenkinsEnv.groovy
index 23e8d1f..2b91359 100644
--- a/vars/jenkinsEnv.groovy
+++ b/vars/jenkinsEnv.groovy
@@ -135,13 +135,21 @@
                 return null
         }
     }
+    /**
+     * https://maven.apache.org/docs/history.html
+     * 3.1.1 and bellow - Java 5
+     * 3.2.x - Java 6
+     * 3.3.1 - Java 7
+     * 3.7.x (planned) - Java 8
+     * for Jenkins - testing only with Java 7+
+     */
     def jdkForMaven(String version) {
         switch(version) {
-            case ~/^3\.0\..+/:
-                return '6'
-            case ~/^3\.[2356]\..+/:
+            case ~/^3\.[012356]\..+/:
             case '3.x.x':
                 return '7'
+            case '3.7.x':
+                return '8'
             default:
                 return null
         }