dev: Update devserver to handle new-style version numbers.

We recently switched from four-token versions like
"0.16.892.0" to three-token ones like "892.0.0".  Image
directories also have prefixes like "R16-" now.

BUG=chromium-os:21018
TEST=added unit tests; also manually tested it

Change-Id: Icc75ad1b4ef227ab6afe7c310c86ed0af6c5bc22
Reviewed-on: http://gerrit.chromium.org/gerrit/8557
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
2 files changed