Run tests on Rails 6.1

[JasonBarnabe]

No description provided.

[coveralls]

Coverage remained the same at 100.0% when pulling 7ebff27 on JasonBarnabe:rails-6.1-test into 333dc27 on iGEL:main.

[JasonBarnabe]

Rubcop complains, unrelated to my changes, that:

lib/it/plain.rb:4:5: W: Lint/MissingSuper: Call super to initialize state of the parent class.
    def initialize(template = '%s') ...
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I'm not sure if Plain needs to inherit from Tag, but the parameters to initialize seem different so calling super doesn't make sense. I've just disabled this cop.

[JasonBarnabe]

@iGEL Can you take a look at this? Thanks.