adapt native build for new gradle versions (targetPlatforms -> model.platforms)

jme3-bullet-native/build.gradle

@@ -50,7 +50,8 @@ sourceSets {
 }
 
 // Set of target platforms, will be available based on build system
-targetPlatforms {
+model {
+    platforms{
 //    osx_universal { // TODO: universal binary doesn't work?
 //        architecture 'x86_64'
 //        architecture 'x86'
@@ -81,6 +82,7 @@ targetPlatforms {
             operatingSystem "windows"
         }
     }
+}
 
 // Defines created C++ libraries
 libraries {