Previously the build script required git to be used and installed which
did not allow to build Multus from a tarball which doesn't contain .git
directory. That made packaging of Multus hard.
Example usage after the change if you do not want to use git:
```
$ VERSION=v3.3 COMMIT=ba33df ./build
```
Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>
This changes introduce goreleaser, which does cross-compile and
package, as well as add version into go code. This change also
changes .travis.yml to allow to other users' forked repo.
