Removed master as default branch_name

The default branch_name from the command line 
invocation is None if no name is passed.

Author: sean-morris

nbgitpuller/pull.py

@@ -303,7 +303,7 @@ def main():
 
     parser = argparse.ArgumentParser(description='Synchronizes a github repository with a local repository.')
     parser.add_argument('git_url', help='Url of the repo to sync')
-    parser.add_argument('branch_name', default='master', help='Branch of repo to sync', nargs='?')
+    parser.add_argument('branch_name', default=None, help='Branch of repo to sync', nargs='?')
     parser.add_argument('repo_dir', default='.', help='Path to clone repo under', nargs='?')
     args = parser.parse_args()