Flush stderr, and formatting

pre_commit_hooks/pretty_format_json.py

diff --git a/pre_commit_hooks/pretty_format_json.py b/pre_commit_hooks/pretty_format_json.py
index b9c0aa4..3967c30 100644
--- a/pre_commit_hooks/pretty_format_json.py
+++ b/pre_commit_hooks/pretty_format_json.py
@@ -124,6 +124,7 @@
                     'File {} is not pretty-formatted'.format(json_file),
                     file=sys.stderr,
                 )
+                sys.stderr.flush()
 
                 if args.autofix:
                     _autofix(json_file, pretty_contents)

tests/pretty_format_json_test.py

diff --git a/tests/pretty_format_json_test.py b/tests/pretty_format_json_test.py
index 3263b76..04e4614 100644
--- a/tests/pretty_format_json_test.py
+++ b/tests/pretty_format_json_test.py
@@ -110,7 +110,8 @@
 def test_diffing_output(capsys):
     resource_path = get_resource_path('not_pretty_formatted_json.json')
     expected_retval = 1
-    expected_out = '''  {
+    expected_out = '''\
+  {
 -     "foo":
 -     "bar",
 -         "alist": [2, 34, 234],