After 1.4.0 is released, merge it back into master so that
1.4.0 is part of the history of master. That means,
  $ git log --first-parent master
will also traverse 1.4.0 and 1.4.0-rc1.

Also, the closest branch parent of master and nm-1-4 branch
becomes 1.4.0 tag.

Also bump the micro version to 1.5.1-dev to indicate that this is
after 1.4.0 is out, otherwise `git describe` uses the 1.4.0 tag.