Fix typo
Update version to 1.4.0.dev

Since plugin devs might not be aware of the bump, at least they'll know that master is now 1.4 after a `bundle update`

/cc @mitchellh
providers/virtualbox: internal networks [GH-2020]
providers/virtualbox: some validation around internal networks
guests/linux: emit upstart event for vagrant mounted if avail [GH-2502]
core: only look for global flags before "--" [GH-2491]
core: create sentinel just before provisioning [GH-2488]