CheckstyleBear: Fetch jar from GitHub

Fetching the jar from sourceforge.net is resulting in
corrupt jar files, and the error reporting and recovery
within coala isnt sufficient for this.

Until the download code is more reliable, fetch the jar
from GitHub.

Related to coala#1461

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')
+            'https://github.com/Berkeley-CS61B/skeleton-sp16/raw/master'
+            '/javalib/checkstyle-6.15-all.jar',
+            'checkstyle-6.15-all.jar')
 
     def create_arguments(
             self, filename, file, config_file,