Update header serialization

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Updates to block header format coming with next reset
• Bumps serialization vector commit (skipping message tests until Implement Dynamic Gas  #615 is done to avoid duplicate work)

Reference issue to close (if applicable)

Closes

Other information and links

[ec2]

lgtm. i had this in my dynamic gas branch, but you got it in ahead :p