feat(version): add -dev flag for unversioned releases

This helps clear some confusion on the current version of Deis
users are currently working with.

[skip ci]
feat(version): add version constant

This utility package is useful for other packages such as deis/deisctl
to know what version of deis they are targeting against.

[skip ci]
Merge pull request #1806 from bacongobbler/dev-version

feat(version): use -dev flag for unversioned releases