Updates readme

- Updates (c) year.
- Updates ruby version to that version in use.
Merge pull request #631 from codeforamerica/minor-readme-update

Updates readme
Performs linting and other general cleanup.

Performs cleanup based on css-linting recommendations, such as:
- removing units from 0 values.
- unseating two or more id selectors.
Removes unused style

Removes `drop-down-container` style, which was unused.
Moves popups to the component style directory

Popups were in the base styles. This commit moves it to it’s own
component style file.