[chocolatey] Change version numbers to something that choco can handle

Because of https://github.com/chocolatey/chocolatey.org/issues/516, the
pre-release part of the version has to start with a character.

Change-Id: I903b63522377225366f1f7467e8878f0dbd3143c
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/143921
Reviewed-by: William Hesse <whesse@google.com>
3 files changed