Fix Nokogiri dep install

[tituspijean]

Problem

User on the forum came up with the following upgrade fail log: https://paste.yunohost.org/raw/zilurobabu

Solution

Follow advice from the log.

PR Status

• [ ] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[tituspijean]

!testme

[yunohost-bot]

:carousel_horse: