Fix syncing down an empty S3 bucket (#783)

* Fix syncing down an empty S3 bucket
* Add test for S3 uploader
* Remove boto.cfg because of https://github.com/travis-ci/travis-ci/issues/7940#issuecomment-310759657
* Specify dummy AWS creds in travis.yml
* Fix S3Uploader in Python 3 and fix test

tests/utils/test_uploaders.py

@@ -0,0 +1,59 @@
+import boto3
+from moto import mock_s3
+from tests.helpers import *
+from CTFd.utils.uploads import S3Uploader, FilesystemUploader, rmdir
+from CTFd.utils import binary_type
+from six import BytesIO
+import os
+
+
+@mock_s3
+def test_s3_uploader():
+    conn = boto3.resource('s3', region_name='us-east-1')
+    conn.create_bucket(Bucket='bucket')
+
+    app = create_ctfd()
+    with app.app_context():
+        app.config['UPLOAD_PROVIDER'] = 's3'
+        app.config['AWS_ACCESS_KEY_ID'] = 'AKIAIOSFODNN7EXAMPLE'
+        app.config['AWS_SECRET_ACCESS_KEY'] = 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY'
+        app.config['AWS_S3_BUCKET'] = 'bucket'
+
+        uploader = S3Uploader()
+
+        assert uploader.s3
+        assert uploader.bucket == 'bucket'
+
+        fake_file = BytesIO('fakedfile'.encode())
+        path = uploader.upload(fake_file, 'fake_file.txt')
+
+        assert 'fake_file.txt' in uploader.download(path).location
+    destroy_ctfd(app)
+
+
+@mock_s3
+def test_s3_sync():
+    conn = boto3.resource('s3', region_name='us-east-1')
+    conn.create_bucket(Bucket='bucket')
+
+    app = create_ctfd()
+    with app.app_context():
+        app.config['UPLOAD_PROVIDER'] = 's3'
+        app.config['AWS_ACCESS_KEY_ID'] = 'AKIAIOSFODNN7EXAMPLE'
+        app.config['AWS_SECRET_ACCESS_KEY'] = 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY'
+        app.config['AWS_S3_BUCKET'] = 'bucket'
+
+        uploader = S3Uploader()
+        uploader.sync()
+
+        fake_file = BytesIO('fakedfile'.encode())
+        path = uploader.upload(fake_file, 'fake_file.txt')
+        full_path = os.path.join(app.config['UPLOAD_FOLDER'], path)
+
+        try:
+            uploader.sync()
+            with open(full_path) as f:
+                assert f.read() == 'fakedfile'
+        finally:
+            rmdir(os.path.dirname(full_path))
+    destroy_ctfd(app)