Update static_header.txt with new license information.

This is the header file for the minified javascript.

zerver/static_header.txt

@@ -1,9 +1,22 @@
 /** @preserve
-  Some of the software provided in this file may be offered under an open source
-  license. If some of the software in this file is offered under an open source
-  license, information about that license will appear in this file.
+  The software provided in this file is offered under a variety of free and open
+  source licenses. Unless otherwise specified, software is provided under the
+  Apache License, Version 2.0, for which the following text applies:
 
-  All other software provided in this file is
-  Copyright (c) 2012-2014 Zulip, Inc. and is provided under the license
-  described in the Zulip Terms of Service: https://zulip.com/terms
+    Copyright 2011-2016 Dropbox Inc. and contributors.
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+
+  If some of the software in this file is offered under a different license,
+  information about that license will appear in this file.
  */