script: Update release-flynn prerequisites

Signed-off-by: Lewis Marshall <lewis@lmars.net>
script: Pull dependent Docker images in release-flynn

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #273 from flynn/release-script-updates

Release script updates
README: Fix typo

[ci skip]

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>