Simplify release process

Release process:
- Edit the code in feature branch, open a PR, merge it
- ~~Add new version number to bower.json and edit changelog.md~~ **No more :)**
- Publish a new release with the appropriate git tag there: https://github.com/guardian/guss-grid-system/releases and document it directly in the new release 

### Changes
- Remove changelog, use GitHub releases to document changes instead
- Remove version number in bower.json: rely on semver tag naming to automatically publish a new version to the bower registry

![ ](http://media.giphy.com/media/qPywh5ABohCHC/giphy.gif)