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having out-of-date versions is not ideal.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]
Shelling out was there for authors convenience, but we are
rather going to have the tag or SHA1 always in RAILS_VERSION
and if the environment variable is blank, then just use
"local" as a reminder that you are just working locally.
The docs server has been updated to set the long SHA1 in
RAILS_VERSION when generating edge guides.
