Fix Buildsystem: Use Travis' Tag Value

Our internal version.gradle values aren't available in the configuration phase so we have to use Travis directly

build.gradle

@@ -26,8 +26,8 @@ repositories {
 }
 
 if (System.getenv('TRAVIS') != null) {
-    if (ext.jmeGitTag != "") {
-        ext.jmeEngineVersion = "3.1.0-" + ext.jmeGitTag + "-SNAPSHOT"
+    if (System.env.TRAVIS_TAG != "") {
+        ext.jmeEngineVersion = "3.1.0-" + System.env.TRAVIS_TAG + "-SNAPSHOT"
     } else {
         ext.jmeEngineVersion = "3.1.0-beta3" // Use Beta 2 instead of 3.1.0-master-SNAPSHOT
     }