fix: analysis is broken when classDir has no .class file but others

Gradle plugin should ignore other files such as .xml, .txt, .properties
and so on. Only .class files should be considered as the target to
analyse by default.

fix #89

src/main/groovy/com/github/spotbugs/snom/SpotBugsTask.groovy

@@ -248,7 +248,9 @@ class SpotBugsTask extends DefaultTask implements VerificationTask {
             if (getClassDirs() == null) {
                 throw new InvalidUserDataException("The classDirs property is not set")
             }
-            return getClassDirs().asFileTree
+            return getClassDirs().asFileTree.filter({ File file ->
+                file.name.endsWith(".class")
+            })
         } else {
             return classes
         }