Update Rails versions [ci skip]

As discussed in #15304, we need to automate this process but for now,
having out-of-date versions is not ideal.

Since master targets 4.2.0, let's also update references to the last 4.1
version to 4.2.0.

Finally, let's remove mentions to versions when this is not needed. The
guides cover the features of the current version anyway.

[Juanito Fatas + Robin Dupret]