CSSMin: version URLs based on content, not mtime

CSSMin: version URLs based on content, not mtime

The content of these files is more stable than their mtimes, which change
every time we roll out a new branch. Because MD5 avalanches well[0], using
the first five hexadecimal digits is sufficient to ensure that the chance
of two successive versions colliding is improbably small (roughly one in a
million).

[0]: https://en.wikipedia.org/wiki/Avalanche_effect

Change-Id: I1bdf94c58786d2545311b238476b48217a5a60af