Block launching the application with Java 9 (#68)

src/main/java/edu/wpi/first/outlineviewer/OutlineViewer.java

@@ -4,6 +4,7 @@
 import javafx.application.Application;
 import javafx.fxml.FXMLLoader;
 import javafx.scene.Scene;
+import javafx.scene.control.Alert;
 import javafx.scene.control.ButtonBar.ButtonData;
 import javafx.scene.control.ButtonType;
 import javafx.scene.layout.Pane;
@@ -18,6 +19,18 @@ public class OutlineViewer extends Application {
 
   @Override
   public void start(Stage primaryStage) throws IOException {
+    if (!"1.8".equals(System.getProperty("java.specification.version"))) {
+      Alert invalidVersionAlert = new Alert(Alert.AlertType.ERROR);
+      invalidVersionAlert.setHeaderText("Invalid JRE Version!");
+      invalidVersionAlert.setContentText(
+          String.format("You are using an unsupported Java version: %s!  "
+              + "Please download Java 8.",
+              System.getProperty("java.version")));
+      invalidVersionAlert.showAndWait();
+
+      return;
+    }
+
     AutoUpdater updater = new AutoUpdater();
 
     PreferencesDialog preferencesDialog = new PreferencesDialog(QUIT, START);