Simplify release process

[kaelig]

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-colours/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