Change file share mode for License.rtf when reading it

This pull request is to fix [MAGN-3879: Dynamo crashed while accessing about page if .rtf was opened in word] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3879)

It crashes because when reading this file, we use ``FileShare.Read``. Interestingly, this flag indicates what *other* openers of the file can do. Details see [this StackOverflow question] (http://stackoverflow.com/questions/897796/how-do-i-open-an-already-opened-file-with-a-net-streamreader). So the fix is to change from ``FileShare.Read`` to ``FileShare.ReadWrite``.