Travis CI builds failing due to gem install error

[andrewdavidwong]

Travis CI builds are failing in https://github.com/QubesOS/qubesos.github.io and https://github.com/QubesOS/qubes-doc due to the following error:

ERROR:  Error installing html-proofer:
	The last version of activesupport (< 6.0, >= 4.2) to support your Ruby & RubyGems was 4.2.10. Try installing it with `gem install activesupport -v 4.2.10` and then running the current command again
	activesupport requires Ruby version >= 2.2.2. The current ruby version is 2.1.0.
Fetching: json-2.1.0.gem (100%)
Building native extensions. This could take a while...
Successfully installed json-2.1.0
1 gem installed
The command "~/qubesos.github.io/_utils/travis.sh" exited with 1.
Done. Your build exited with 1.