Remove incorrect assumption that output will be in folder named classes

When running in Eclipse, by default Gradle builds its output into a
folder named bin. This commit update the annotation processor to remove
the failure assumption that the output will always be located beneath
a folder named classes.

Closes gh-2369
See gh-2361

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessor.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -393,9 +393,11 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 			// Gradle keeps things separate
 			String path = file.getPath();
 			int index = path.lastIndexOf(CLASSES_FOLDER);
-			path = path.substring(0, index) + RESOURCES_FOLDER
-					+ path.substring(index + CLASSES_FOLDER.length());
-			file = new File(path);
+			if (index >= 0) {
+				path = path.substring(0, index) + RESOURCES_FOLDER
+						+ path.substring(index + CLASSES_FOLDER.length());
+				file = new File(path);
+			}
 		}
 		return (file.exists() ? new FileInputStream(file) : fileObject.toUri().toURL()
 				.openStream());