Avoid a compilation issue when using IntelliJ

Using a TreeMap causes a ConcurrentModificationException,
whereas the ConcurrentSkipListMap has no such issue.

Author: kmader

src/main/java/org/scijava/annotations/AbstractIndexWriter.java

@@ -44,6 +44,7 @@
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.TreeMap;
+import java.util.concurrent.ConcurrentSkipListMap;
 
 import javax.lang.model.element.AnnotationValue;
 
@@ -60,7 +61,7 @@
 public abstract class AbstractIndexWriter {
 
 	private final Map<String, Map<String, Object>> map =
-		new TreeMap<String, Map<String, Object>>();
+		new ConcurrentSkipListMap<String, Map<String, Object>>();
 
 	protected synchronized boolean foundAnnotations() {
 		return !map.isEmpty();