Ensure Android compatibility (#25)

* Fixed a line of code using a construct that is not available in early Android SDK versions.

This causes crashes in Android with SDK < 24.

* Update gradle.properties

Co-authored-by: Michael Akinde <michael.work@akinde.dk>

src/main/java/com/bladecoder/ink/runtime/Flow.java

@@ -30,7 +30,13 @@ public Flow(String name, Story story, HashMap<String, Object> jObject) throws Ex
 		// choiceThreads is optional
 		Object jChoiceThreadsObj;
 
-		jChoiceThreadsObj = jObject.getOrDefault("choiceThreads", null);
+		// This does not work in Android with SDK < 24
+		//jChoiceThreadsObj = jObject.getOrDefault("choiceThreads", null);
+		if (jObject.containsKey("choiceThreads")) {
+			jChoiceThreadsObj = jObject.get("choiceThreads");
+		} else {
+			jChoiceThreadsObj = null;
+		}
 
 		loadFlowChoiceThreads((HashMap<String, Object>) jChoiceThreadsObj, story);
 	}