HTML-escape text; implement ‘pre’ element; tests; updates

Main features:

Add support to join multiple blocks in asText() with any string, not only one whitespace as proposed in PR prismicio/prismic-richtext#8 (it makes sense to release that prismic-richtext PR first, but if it isn't it shouldn't break anything, the second arg will just be ignored).
HTML-escape text content to fix #22. An external dependency escape-html was added. It is very small. I locked the version to 1.0.3 to minimise the risk of breaking anything in the future. I also checked that it actually is really fast. In my opinion the dependency is justified.
Implement the preformatted element (<pre>) to fix #24.
Added a few very basic tests (better than nothing).

Included the minor change from PR #23 (Update Babel dev dependencies), which makes #23 redundant.
Updated .travis.yml…
- to run the newly added tests
- added Node version 8
- removed macOS as I believe it doesn't add any value in this case and only doubles build times (comments?)
- removed global installation of NPM package tape as it looks like it's redundant.
- replaced npm run with yarn run because everything else is run with yarn too.
Updated package.json…
- added the test script
- changed prismic-richtext dependency from version 0.2.1. to latest 0.4.1
- updated Babel dependencies (as in PR #23)

I know, I really should have broken everything down into more PRs, but actually it's not that much.