Fix small issue with Gradle when the TRAVIS_TAG is not set

build.gradle

@@ -30,7 +30,7 @@ repositories {
 }
 
 if (System.getenv('TRAVIS') != null) {
-    if (System.env.TRAVIS_TAG != "") {
+    if (System.env.TRAVIS_TAG != null) {
         // v3.2.0-stable-sdk1 becomes 3.2.0-stable
         // Intentionally not using lastIndexOf here, we want the second "-".
         ext.jmeEngineVersion = System.env.TRAVIS_TAG.substring(1, System.env.TRAVIS_TAG.indexOf("-", System.env.TRAVIS_TAG.indexOf("-") + 1));