Update eclipsefdn-hugo-solstice-theme to latest version (0.0.17 => 0.0.21)

Describe the bug After upgrading the eclipsefdn-hugo-solstice-theme module to latest version (0.0.17 => 0.0.21), Travis fails to build the server using Hugo. Local instances still run properly. Downgrading to the version of Hugo used by Travis for the site (0.46) sees the bug reproduce locally.

To Reproduce Steps to reproduce the behavior:

1. Retrieve a Hugo install for version 0.46 and install it locally.
2. Upgrade eclipsefdn-hugo-solstice-theme in the package.json file in the root of the site.
3. Remove the npm artifacts and reinstall them.
4. Build the site using Hugo 0.46

Expected behavior The build should succeed.

Additional context The fix here is to upgrade the old version of Hugo to something more modern. This will be done when there are more resources available to cover any potential issue that arises from the upgrade.