Update to NEW sha256 checksum of Vagrant 1.7.0

There were some problems with the initial packages, and Vagrant 1.7.0 has
been re-packaged with same version number.

See https://github.com/mitchellh/vagrant/issues/4918.

Fix https://github.com/caskroom/homebrew-cask/pull/7969#issuecomment-66505058