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]

Author: Matthew Fisher

docs/contributing/releases.rst

@@ -36,6 +36,7 @@ github.com/deis/deis Repo
       click "Edit".
 - Commit and push the deis/deis release and tag
     * remove "-dev" from __version__ fields in Python packages
+    * remove "-dev" from Version constant in version/version.go
     * ``git commit -a -m 'Updated for vX.Y.Z release.'``
     * ``git push origin release``
     * ``git tag vX.Y.Z``
@@ -53,6 +54,7 @@ github.com/deis/deis Repo
 - Switch master to upcoming release
     * ``git checkout master``
     * update __version__ fields in Python packages to *next* version + "-dev"
+    * update Version constant in version/version.go to *next* version + "-dev"
     * ``git commit -a -m 'Switch master to vA.B.C.'`` (**next** version)
     * ``git push origin master``
 

version/version.go

@@ -0,0 +1,3 @@
+package version
+
+const Version = "0.12.0-dev"