Refactor buildSpriteString and update sprite docs

Updated .travis.yml to match the example provided in the documentation
https://semantic-release.gitbooks.io/semantic-release/content/docs/recipes/travis.html

chore(dependencies): Bump eslint-plugin-prettier from 2.4.0 to 2.5.0

chore(dependencies): Bump semantic-release from 8.2.0 to 12.2.2

Bumps [eslint-plugin-prettier](https://github.com/prettier/eslint-plugin-prettier) from 2.4.0 to 2.5.0.
- [Changelog](https://github.com/prettier/eslint-plugin-prettier/blob/master/CHANGELOG.md)
- [Commits](https://github.com/prettier/eslint-plugin-prettier/compare/v2.4.0...v2.5.0)

Bumps [semantic-release](https://github.com/semantic-release/semantic-release) from 8.2.0 to 12.2.2.
- [Release notes](https://github.com/semantic-release/semantic-release/releases)
- [Commits](https://github.com/semantic-release/semantic-release/compare/v8.2.0...v12.2.2)

chore(dependencies): Bump html-minifier from 3.5.7 to 3.5.8

chore(dependencies): Bump eslint from 4.13.1 to 4.15.0

Bumps [eslint](https://github.com/eslint/eslint) from 4.13.1 to 4.15.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v4.13.1...v4.15.0)

Bumps [html-minifier](https://github.com/kangax/html-minifier) from 3.5.7 to 3.5.8.
- [Release notes](https://github.com/kangax/html-minifier/releases/tag/v3.5.8)
- [Commits](https://github.com/kangax/html-minifier/compare/v3.5.7...v3.5.8)

Bump eslint-plugin-prettier from 2.3.1 to 2.4.0

Bumps [eslint-plugin-prettier](https://github.com/prettier/eslint-plugin-prettier) from 2.3.1 to 2.4.0.
- [Changelog](https://github.com/prettier/eslint-plugin-prettier/blob/master/CHANGELOG.md)
- [Commits](https://github.com/prettier/eslint-plugin-prettier/compare/v2.3.1...v2.4.0)

Bump lint-staged from 5.0.0 to 6.0.0

Bump eslint-config-airbnb-base from 11.3.2 to 12.1.0

Bumps [eslint-config-airbnb-base](https://github.com/airbnb/javascript) from 11.3.2 to 12.1.0.
- [Commits](https://github.com/airbnb/javascript/compare/eslint-config-airbnb-base-v11.3.2...eslint-config-airbnb-base-v12.1.0)

Bump eslint from 4.10.0 to 4.13.1

Bumps [eslint](https://github.com/eslint/eslint) from 4.10.0 to 4.13.1.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v4.10.0...v4.13.1)

Bump @commitlint/cli from 5.0.0 to 5.2.5

Bump eslint-config-prettier from 2.7.0 to 2.9.0

Bumps [lint-staged](https://github.com/okonet/lint-staged) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/okonet/lint-staged/releases/tag/v6.0.0)
- [Commits](https://github.com/okonet/lint-staged/compare/v5.0.0...v6.0.0)

Bumps [eslint-config-prettier](https://github.com/prettier/eslint-config-prettier) from 2.7.0 to 2.9.0.
- [Changelog](https://github.com/prettier/eslint-config-prettier/blob/master/CHANGELOG.md)
- [Commits](https://github.com/prettier/eslint-config-prettier/compare/v2.7.0...v2.9.0)

Bump webpack from 3.8.1 to 3.10.0

Bump html-minifier from 3.5.6 to 3.5.7

Bumps [html-minifier](https://github.com/kangax/html-minifier) from 3.5.6 to 3.5.7.
- [Release notes](https://github.com/kangax/html-minifier/releases/tag/v3.5.7)
- [Commits](https://github.com/kangax/html-minifier/compare/v3.5.6...v3.5.7)

Bump husky from 0.13.4 to 0.14.3