In the original manifest updating process, the file was truncated first
before was written with new contents. There is a small chance that
the web server may read back empty file or incomplete file.
The new code makes the update operation atomic (by writing to a temporary
file first then replace the old file with the temporary file).
Fixes#186
Change-Id: I2fd564cb12b922b032c0e9f70d2132a5b12ff098