CheckstyleBear: Use checkstyle 7.6

Upgrade checkstyle from 6.15 to 7.6

Also rename the cached jar to match the downloaded jar file,
so that it is more recognisable for someone investigating
any problems, and so the filename includes the version, so that
the cache can be shared with multiple versions of CheckstyleBear.

Closes #1465

bears/java/CheckstyleBear.py

@@ -53,9 +53,9 @@ class CheckstyleBear:
 
     def setup_dependencies(self):
         type(self).checkstyle_jar_file = self.download_cached_file(
-            'http://sourceforge.net/projects/checkstyle/files/checkstyle/6.15'
-            '/checkstyle-6.15-all.jar',
-            'checkstyle.jar')
+            'http://sourceforge.net/projects/checkstyle/files/checkstyle/7.6'
+            '/checkstyle-7.6-all.jar',
+            'checkstyle-7.6-all.jar')
 
     def create_arguments(
             self, filename, file, config_file,