Replace bumper with grunt-bump, closes item:519

[Usage][]:

```bash
grunt bump[:[patch|minor|major|build|git]]
```

Note, this will also automatically push to `origin`.

If this gets merged, the [releases wiki page][1] needs updating.

[1]: https://github.com/eHealthAfrica/LMIS-Chrome/wiki/Releases#tagging
[usage]: https://github.com/vojtajina/grunt-bump#usage-examples