Update dependencies including security incident

This is the regular batch update for outdated dependencies and actually
the first one for this project.

Anyway, it is also a important update since it includes updates for the
hijacked package event-stream (1) that includes malicious code which has
been revealed yesterday (2). Nord Docs is affected passively because the
package is a a traverse dependency of the used package npm-run-all (3).
The maintainer has reacted quickly and released the fix version
4.1.55 (4).

Note that packages marked with `<!>` have been affected by the security
incident!

>>> Production Dependencies

- gatsby `2.0.50` -> `2.0.55` (5)
- gatsby-source-filesystem `2.0.8` -> `2.0.9` (6)

>>> Development Dependencies

- husky `1.1.4` -> `1.2.0` (7)
- lint-staged `8.0.5` -> `8.1.0` (8)
- npm-run-all `4.1.3` -> `4.1.50` (4) <!>

References:
  (1) https://www.npmjs.com/package/event-stream
  (2) https://snyk.io/blog/malicious-code-found-in-npm-package-event-stream
  (3) https://www.npmjs.com/package/npm-run-all
  (4) https://github.com/mysticatea/npm-run-all/releases/tag/v4.1.5
  (5) https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/CHANGELOG.md#2055-2018-11-22
  (6) https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-filesystem/CHANGELOG.md#209-2018-11-26
  (7) https://github.com/typicode/husky/blob/master/CHANGELOG.md#120
  (8)  https://github.com/okonet/lint-staged/releases/tag/v8.1.0

Resolves GH-47