build.gradle: upgrade testFX dependency to 4.0.15-alpha

The current testFx dependency we are using is 4.0.12-alpha, which gives
the error below when testing with JDK 11:

java.lang.RuntimeException
   Caused by: java.lang.NoSuchMethodError

TestFX adds support for JDK 11 since version 4.0.14-alpha.[1]

As we are planning to use JDK 11+ as our supported language, let’s
upgrade the testFX dependency to its latest stable version 4.0.15-alpha
to solve the problem.

[1]TestFX/TestFX#640

build.gradle

@@ -61,7 +61,7 @@ if (SystemUtils.IS_OS_WINDOWS) {
 }
 
 dependencies {
-    String testFxVersion = '4.0.12-alpha'
+    String testFxVersion = '4.0.15-alpha'
     String jUnitVersion = '5.1.0'
 
     compile "org.openjfx:javafx-base:11:${platform}"