Update version to use -dev suffix

According to the plan listed in docs/releasing.md.

The gitMinor will keep using a "+" suffix instead for now.

Added a //TODO to deprecate gitMajor and gitMinor in a follow up.

Tested:
- Built it from the git tree:

    $ make
    $ _output/go/bin/kubecfg -version
    Kubernetes version 0.2+, build 8d31eb03c11d4db64ae26809eef7f73070efd811
    $ _output/go/bin/kubecfg -version=raw
    version.Info{Major:"0", Minor:"2+", GitVersion:"v0.2-29-g8d31eb03c11d4d", GitCommit:"8d31eb03c11d4db64ae26809eef7f73070efd811", GitTreeState:"clean"}

- Built it with a direct `go install` (same as tarball):

    $ GOPATH=${PWD}/_output/go:${PWD}/Godeps/_workspace go install $KUBE_GO_PACKAGE/cmd/kubecfg
    $ _output/go/bin/kubecfg -version=raw
    version.Info{Major:"0", Minor:"2+", GitVersion:"v0.2-dev", GitCommit:"", GitTreeState:"not a git tree"}
    $ _output/go/bin/kubecfg -version
    Kubernetes version 0.2+, build (unknown)

A follow up commit will address the output of the -version (without "raw") command to use gitVersion instead of Major + Minor.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #1247 from filbranden/version_dev

Update version to use -dev suffix